Mostly because they are really, really good, and they don't ask much for it.
I think I pay them less than $150 for a yearly subscription to their entire toolset, which is a price point that makes it stupid not to buy them. ReSharper is well worth a cup of coffee a week, by itself, and then there is DataGrip, WebStorm, IntelliJ, profilers, decompilers, and other stuff thrown in.