I feel the same way, Marth is prob my 2nd or 3rd most played character. I find I land tippers often and since I've played Marth at the very least like a secondary in Melee, Brawl, PM and Sm4sh maybe I just have experience with the character. Then again I guess I'm not playing top pros so maybe spacing against pro level opponents is much harder but I honestly felt all the Lucina secondaries were kind of a cop out, like they weren't practiced enough to properly space with Marth so they just go Lucina. Especially after I saw ZeRo's rant about Marth's bair. *insert salt here*
I think when people think of Marth tippers they fixate on his f-smash bc that's his "early KO tipper move" but it's not his only move that benefits from tippers. IDK if you can even say landing tippers is harder in doubles bc if you have synergy you can combo with a teammate with jab locks, footstools, and grabs and get tipper kills.
I also think sour spot combos are underdeveloped in the Marth meta. You get your opponent to certain percents and every Lucina move will knock an opponent away too far and make follow-ups more difficult. (In Melee Marth's Ken Combo uses a sour spot fair followed by a dair so there's potential for sour spots to be useful)
"Marth gets exclusive combos because of Sourspots" is a very old argument. What people are forgetting on the other side of the coin is that Lucina also just so happens to have exclusive combos for her due to how her sword works.
People have been labbing Marth's sourspots and the potential combos it can do very well since he was last buffed, actually. People have found plenty of stuff, but Melee/Brawl level sourspot combos will simply never exist in this game just simply due to how this game works in general and how very little hitstun Marth's sourspots have in this game (I saw stale Sourspot Up Air be
unsafe on hit once with Marth), alongside how horrible the FAF in Marth's moves are in general here.
Then again I guess I'm not playing top pros so maybe spacing against pro level opponents is much harder but I honestly felt all the Lucina secondaries were kind of a cop out, like they weren't practiced enough to properly space with Marth so they just go Lucina. Especially after I saw ZeRo's rant about Marth's bair. *insert salt here*
This is actually a kind of mentality we from the Lucina community just really don't like witnessing: seeing Lucina as "cop-out Marth".
Using ZeRo again as an example, on his streams he has shown time and time again that he is more than qualified to play Marth if he wanted. He just doesn't want to. He doesn't like him. He prefers to play Lucina, he finds her far more consistent and also on his streams he implied he values this consistency far more than whatever tipper has the potential to offer him. MKLeo supposedly saying tippers feel random is something ZeRo said Leo told him, and also according to him, whenever Leo practices with him, the outcome of the match seems to be dictated way too much by whether or not Leo lands a tippered kill move, which leads to the extreme outcomes of "Marth destroying his opponent" or "Marth getting destroyed without taking a stock", which further drove him to playing Lucina.
You see, Lucina
has lower highs, yes, but she also happens to have much lower lows. She's just safer. You still retain your ability to occasionally KO somebody at 50-80% with FSmash at the ledge (yes, her FSmash is still that strong) or with a Shield Break punish at 30-40, while also having the ability to still kill well and not needing tippers if you get to more common KO% (90-140);
Also, some of these comments of yours kind of seem to imply that to play Lucina, you don't need to "properly space"? You still need to space just as much as him. She has a long sword so you need to use it to its fullest extent to keep people out of you. The only real difference in spacing they have is what I mentioned earlier in kill moves, where it's much easier to get kill moves with her and get them to kill more consistently because you don't need to worry about hitting them at the tip of the blade. But that's still more a matter of preference than it is somebody "not being able to space their moves".
Lucina is more suitable as a secondary because of these traits, but don't be mistaken. Just because she is "easier" than Marth, that doesn't make her worth any less, and it also doesn't even make her an "easy" character period.
Also, yes, landing tippers in doubles is way harder. Tippers require tight spacing and patience, and no amount of that will fix the incredibly hectic pace that a match with 3 other people generates. When it's time to kill in doubles, Tippering FSmash and Up Smash is way too difficult out of throws, and Lucina's versions of those moves happen to hit like a truck at 200KM/H regardless of where the opponent was thrown, and that really pays dividends in matches. That's not the only example, obviously. If an opponent gets up thrown by your teammate at kill percent and you need to rush to Up Air them, you don't need to aim for the tipper and you will still kill, while on a same situation with Marth, you either need to waste time spacing yourself for the tipper and risk the opponent popping out of hitstun while you're at that or content with the sourspots (and also not getting the kill, which was the entire point). There's also the fact that on 1v1, when team comboing somebody, Lucina's hits deal very high damage and send at a friendly distance to keep the combo going very well. The sourspots deal too little damage and might not knock back far enough for your teammate to safely continue the combo and juggle the opponent back to you, while the tippers send people too far and too fast and might make the team lose the rythm of the string they're attempting (especially if you get an unexpected one).
This all just leads back into the question you posed at the title: Why do people play Lucina over Marth?
Preference.
Just that.
I personally started playing her over Marth simply because I liked her better than Marth. Nowadays. I just really prefer the way she handles and the way I can kill with her. I prefer her combo game and I like the stage control I can get with my hits regardless of where I hit them. I also like 2-framing people at the ledge with my FSmash and then KOing them sub 65 and just watching them complain how "broken" that move is. Or landing DB1>Nair1>FSmash and complaining how broken that string is.
She has her own worth over Marth. That is what you need to see.