First, Kindle Oasis has a great form factor. I want a reader that is shaped conveniently to hold with one hand and flip pages without having to lift my thumb. Oasis does exactly that.

Second, I find that Amazon is the most likely to have books that I care about. Other catalogs aren't as extensive, and depending on your preferences, this can get really noticeable.

Lastly, I already have a very large Kindle library from way back, and switching to a different incompatible device means abandoning all of that.