Why? Swellow is one of the coolest pokemon introduced in Hoenn. Swellow is the kind of bird who doesn't mess around, and this is proved by its official art. Swellow is also (iirc) the fastest Pokemon of Gen 3 besides the Deoxys forms, clocking in at an insane base 125 speed, while also having a decent 85 attack stat and utility in Fly, Quick Attack and some powerful moves like Return.

So why?

Not really sure what you're talking about... As far as I know, Swellow has always been well-loved. I've seen it be people's #1 bird since 2004. And it was rather popular in Gen 6 NU as well with a really cool ability.

Everyone has their preferences. I like Swellow, but from the beginning I liked Taillow a bit more as it's rather cute and quite strong for an early route Pokemon, so I used both of them and they were useful in contests too.

I think it's because people think Swellow's design is uninteresting. It does barely change compared to Taillow, in fact it's only a bigger and more badass Taillow on paper.

Unfezant is lame, you're always better with an Eviolite Pidove.

I think Swellow is pretty cool and it's always on my Hoenn teams, but I can see how it would get overshadowed by Talonflame, Pidgeot and Staraptor which are reasonably comparable to it as early game birds.

It would surprise me if there were any specific dislike of Swellow, it's 100% inoffensive.

This. Please do inform us where you heard about people disliking it.

I don't dislike Swellow at all, although I've never personally used one. Although I can see it getting overshadowed by other early bird Pokemon, plus the fact it only evolves once instead of twice like other early birds is a reason I could see people being turned away from it.

http://notallpokemonareequal.blogspot.ro/2011/09/no-277-swellow.html

That's just 2 people disliking it.

Because he's a physical attacker. I've always found special attackers are the most useful in Pokemon games. Usually with Ice Beam.

I didn't know that there were people who don't like Swellow lol

I know right, I have never heard anyone say they don't like it. If anything I thought people would love it because the whole 'thunder armour' scene in the anime.

And it's also the best tier. 8D

In my opinion, Swellow is one of the best flying-type pokemon in R\S\E but I prefer other flying-type pokemons such as Gyrados since some of them have 2 types which can take advantage on opponent

I understand that Swellow might be overshadowed by Talonflame, Staraptor, and Pidgeot, but I dont understand why people would dislike the pokemon.

I mean, I like Swellow just fine. It's not really one of my favorite Pokémon, but I've used it a few times. I actually caught and used a shiny Taillow once; he was pretty awesome.

The Taillow line actually kinda reminds me of the Spearow line in that they're both grouchy, short-tempered little early-game birds with just two total stages of evolution.