Sadly, he's actually insanely popular in Japan (I'd say his Japanese voice actor likely has a lot to do with that).
Plus, people basically made a meme out of him thanks to his quote from Robin/Lucina's trailer.
https://events.fire-emblem-heroes.com/vote/result
https://events.fire-emblem-heroes.com/vote2/result
In the male's division, Chrom is 4th in the first poll and 6th in the second one (these are from the FE Heroes polls). So okay, he's popular.
Still, if FE was getting another Echo I would've preferred it to be from a game that isn't already represented by 2 characters in Robin and Lucina.
Also, Lyn's placing in the first Choose Your Legends poll is telling; she got a ridiculous number of votes, even more than Lucina and the rest of the Fateswakening gals. She isn't in the second one because they removed the top 2 males and top 2 females from the first poll.
I mean, he was THE popular suggestion for Awakening ever since the start of Smash 4 speculation
I was talking about his popularity within FE circles, not Smash circles. But I have been shown to be wrong on that front anyway. Lucina is more popular than him in general, though.
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Anyway, I should probably get back to talking about Isaac and GS lol.
I can get if Sakurai isn't the biggest fan of the series but I don't get why any of this makes him an opposer of a playable character from the series.
It's not so much that Sakurai would oppose having a GS character, just that he has no particular interest in representing GS and since the series is dormant he has little motivation to do so. If GS was "relevant" like Animal Crossing at the time of Smash 4, Sakurai might be more compelled to give it a PC just like he did Villager after deciding in Brawl that Villager wouldn't work. Many of the "dead" retro characters come from the 80s, which is the era Sakurai is most familiar with. Xenoblade, in contrast to GS, is a series Sakurai has been shown to enjoy so even though Xenoblade didn't sell too great Shulk still got in 4 (of course, this isn't the only reason Xenoblade got represented but it very likely was a contributing factor). Sakurai even admitted that Shulk was the type of character that didn't usually make it into Smash. You could also say that Xenoblade was more "relevant" at the time of Smash 4 than GS was, so it's not clear-cut, but the idea is that if Sakurai had more personal investment in GS we might have had Isaac by now.