Reason for the cancellation of show

Easy answer to an easy question: Disappointing number of viewers. FX had higher expectations, so they cut it down to a final season. Certainly not the worst case scenario, especially if you have in mind that they originally had 3 season in mind.
I can live with that.

And while the show might have been disappointing from a corporate standpoint, it certainly wasn't bad either. Otherwise they would have just shut down the show right after S3.

I remember them originally saying five seasons. But, there were three books. And Season 1 covered pretty much all of Book 1, and then a few things earlier. So I think it's fine to end after Season 4. I'd really say it's more for creative reasons; they're at the end of the story, and doing more would be stretching it and risk cancellation before actually finishing the story. And not to be caught up in semantics, but I don't think the term cancellation really applies here. The show was renewed for a final season while their penultimate season still had a month of airing left. I think a cancellation is more when it happens by the network right near the end of what ends up being the final season, or right after, so they don't have an adequate chance to wrap up their story. This is just a show going out on its own terms really.

They're trying to end it properly, when the story ends, instead of dragging it out forever like too many shows.

Del Toro said the original idea for this whole thing was a TV miniseries, so it seems fitting.

its ending? too bad

Same reason Game of Thrones is ending in a year or 2- because they got through all the books. I believe this was the reason given for why they decided this would be the last season.

They could have made this show 10x better with a very simple formula. More Quinlan, more action. Seeing Quinlan being a badass is one of the only parts of the show I really enjoy.

progressively ... it's been terrible from the start.

It wasn't cancelled. It ended. The show was originally only supposed to be three seasons - one for each book. They talked Del Toro into allowing them to split the final book into two seasons based on the strong performance of season 1.