A lot of people that play Pokemon have played it since the first generation. You maybe heard of "genwunners".
Those people like to hate on Pokemon that exist after the first and second gen because they think the first ones are the best. It goes beyond personal preference.
Gen V had the most Pokemon based on inanimate objects( I think) so it got more hate than other generations. Mostly because of Vanilluxe, Garbodor and such.
This doesn't mean everyone that played Pokemon since gen 1 is like that, just some.
To be honest they were kind of running out of ideas, or so it seemed, but it got more hate than necessary.
This doesn't mean I'm 100% correct but it's definitely one of the major reasons.
Hope this helps.