Ivysaur has range and KO potential in her smashes. Her A combo is nice because it spaces your opponents away and her F-Tilt racks nice damage,is really quick, and it has good priority. I think people don't like her too much because she's pretty laggy, Razor Leaf is not THAT spammable because it's a Side-B special, instead of a Neutral-B special, and a passive-aggressive Up-B that's a teether recovery (even though the radius of it's edge grabbing range is quite large). I believe players have to have better timing, spacing, and more tactics with Ivysaur, more so than the other two.
Charizard has powerful smashes, range, speed, and power with his tilts, three jumps + Up-B + Glide, Rocksmash is deadly close range and it has many hitboxes that can disrupt an opponents attack, he can get free damage with flamethrower, has insane grab range and speed, and his D-Throw can kill at around 130% or more.
Squirtle is very agile and has a lot of speed. His aerial attacks have insane priority and virtually no lag afterwards, he can combo instantly from an aerial attack just by tapping A and the combo's last attack has some decent knockback (for a combo anyways), his Side-B can be used for mind games, annoying others in FFA, and be used as horizontal recovery when knocked out. He has a powerful Up-Smash and a D-Throw that has an identical KO potential as Charizard's. Squirtle also has good grab range as well.
Yes I know, I didn't talk about the other two's flaws because I just wanted to state why people lean away from Ivysaur. Charizard is a powerhouse and a tank. Squirtle is very fast and dominates the aerial game. Because of those factors, I believe it's easier to pick up the other two than Ivysaur.