As @Dan comments, this issue has been covered on English Language & Usage. But we can't close cross-site duplicates, so I'll reproduce the essence of his answer there...

Short answer: it's not the istic, it's the underlying ist, or ultimately ism.

Obviously not all -isms are negative (there's optimism as well as pessimism). But an awful lot are, and that association carries over to -istic forms. There's usually a valid "root" form (simple, traditional) forcing us ask what "tweak" to the meaning of the root word is intended by simplistic, traditionalistic? By default, unless anything suggests otherwise, we assume the intention is to disparage an -ism.

As @Dan comments, this issue has been covered on English Language & Usage. But we can't close cross-site duplicates, so I'll reproduce the essence of his answer there...

Short answer: it's not the istic, it's the underlying ist, or ultimately ism.

Obviously not all -isms are negative (there's optimism as well as pessimism). But an awful lot are, and that association carries over to -istic forms. There's usually a valid "root" form (simple, traditional) forcing us ask what "tweak" to the meaning of the root word is intended by simplistic, traditionalistic? By default, unless anything suggests otherwise, we assume the intention is to disparage an -ism.