After reading the title, the first response in my head was "because they are stony corals" but I'll leave the smart-alec comments off the thread
I would say you have a combo lighting and flow issue. increasing both may help greatly. The rest looks good parameter wise.
I would also have to say to start making your own saltwater. You never know what you are going to get at the LFS. It's an unfortunate truth in the hobby that all LFSs are different. Yours may be good, but if you have issues, at least if you make your own, you have full control over it.