- It's probably from association with the film not being very good, or indicatve of what Tom & Jerry were about. There's very little of the slapstick that made the original shorts popular, and since the talking and singing take up so much of the movies time, it isn't unfair to assume that the humour of the source material got stripped out in favor of it. Ergo, the talking and singing are bad because they replaced what made the theatrical shorts appealing to begin with.
- Doug and Rob Walker explained it best in their commentary track: "They inherently aren't talking characters. Just because Mr. Bean gives a little speech in an movie, that does not make him a 'talking character'".
- Could be the Negative Continuity that the movie implies about the past scenes where they've spoken/sung. The movie makes it clear that neither knew the other could talk, but instead of the few short lines and songs, they talk for the entire movie. Basically, it seems like the movie was disregarding all the old cartoons and treating it like a new gimmick, which pushed away long-time fans.
- It's a Misblaimed part. The movie very much seems to have thrown Tom and Jerry into a project where two completely new characters may have done better. This is what probably angers most of the T&J public, but the whole "they talk?" thing is overblown by a much smaller but much louder team. The internet no doubt helped that group seem larger than they are, since after all if those people had watched more Tom and Jerry over the years, them talking, singing and being friends are all more Mythology Gag material than insults.
Headscratchers / Tom and Jerry: The Movie
Why is the fact that Tom and Jerry talk (and have musical numbers) such a Berserk Button for the fandom? Yeah, they never held conversations before, but in the theatrical shorts, they talked occasionally and sang all the time. It's not like this was an Ass Pull on the filmmakers' part.