When it comes to recency picks, they actually make sense beyond "yeah, they're recent."
The thing is? Smash is an absolute advertisement. What does that also mean? A lot of newcomers will make people interested in their series. How does that matter? It means that if you want to play as the character in their home game, you need to be able to purchase said game in some way. This also means if said game doesn't exist outside of emulator, the advertisement factor is more hurtful towards the company. So making sure to have some more recent picks instead of just older characters is very good for business in itself.
Think of it this way; Nintendo does not like people pirating games. Sakurai is well aware of these kind of things to begin with. Like, it's common knowledge. This means that by picking lots of characters with no releases, Nintendo now has to suddenly focus more on old products being released... but some aren't even in the West, now are they? They now have to spend time and money to translate some of these games. And some they can't throw on without licensing issues either or other various problems(Hint: They don't fully own a lot of games on their systems. Certain rights, even for a Nintendo exclusive, are tied to another person. It's not a coincidence Mother 3 isn't in English yet. There's a lot they need to figure out if they ever want to re-release it. EarthBound had similar issues too). Let's also note this is part of why it was even considered for Marth and Roy to not come overseas at the time, or for that matter, why Ness was deliberately kept in Brawl, as they couldn't bring Lucas in alone when they can't even have an overseas sale.
This doesn't adhere to every single character either. It's just more of an attempt to pick closely mostly recency picks to make sure business is kept well. Also, it should be noted that every single third party did have an active product in some way you could obtain, or was really easy to do without pirating. It may have also came out a bit later, but they still existed somewhere. They're all business decisions. A completely dead third party(you can't count B&K when you were able to play them on the latest Xbox in some way) is a very bad pick if nobody can make even a tad of money off of its advertisement factor. It seems cheeky, but it's the only way a crossover like this can be easily worked with.
I want more picks from older stuff too. And some aren't going to have that issue. Most Pokemon picks are clearly possible to get games related to said Pokemon without issues. Fire Emblem is not the case in the same way. Some characters do not have games out, and FE Heroes isn't exactly the best way to make it work. It's still an awesome thing to have exist(since it overall is a great way to show off tons of cool characters). Promotional Picks aren't understandably not that popular, but it overall is the best approach to make sure everything goes smoothly as much as possible. As long as pirating exists, what is basically a series that does high advertising absolutely will try to make sure said piracy is unlikely. In fact, the "timing of who gets in" is literally based around having an active product so said character at least advertises something you should be able to find. Respectively, a lot of other fighting games don't worry as much about that, outside of Crossovers, and they clearly are less advertisey than Smash inherently(they still do newer characters too, of course). It's just not the same effect since said games also rotate members and cut far more frequently than Smash dares to. That's kind of a big deal.
