I would say probably because of their "assault" rifle stigma. There was the shooting some years back where several officers was killed with someone using a Mini 14. I would think that is some of a detractor, whether or not its fair is up in the air.
I can't vouch for their mini 14, but their 22's are fine rifles that are fun to shoot.
the poor accuracy stigma has followed them through the years. Add to that the fact that the factory has always limited thier hi caps to law enforcement and you have a mediocre reputation.
I've owned 2 (one 188 series and one 580 series) and they have both been very acceptable shooters. I have never had a single malfunction that wasnt related to aftermarket magazines either. Now I only use factory mags when it counts.
The new model 580 series is a great gun and you can expect to see 2" groups with decent ammo. If one were to add one of the clyde armory socom railed aluminum stocks like the m14, a flashhider and factory mags, and you'd have one of the most reliable capable 556 guns on the market. I'd put it up against a stock sig, AR, Masada, and XCR in a long term shootout. It may not win, but it'll do better than folks give it credit for
If I could change two things it's be to add a chromelined bore and a threaded barrel from the factory.