Slackware (the first distribution on which I ran webhosting) and Arch Linux are both aimed toward sophisticated linux administrators with specific aims and skillsets. Neither are aimed at shared webhosting users, and neither are supported by DirectAdmin. DirectAdmin is aimed at system administrators who want to be able top offer shared webhosting without having to make every choice and decision and installation.
Big difference.
I've been hosting websites for others since 1995 (no one did before that; the Internet was entirely US Govt operated before that) and there were lots of discussions in the earliest days of whether or not we should automate at all; the concensus ended up to let others make the decisions and automate the work, so we could concentrate on maintaing platforms that could just work.
But to me the biggest issue is the selling point. I can either let my prospects know we use MySQL (which they've all heard of, and which virtually all site platforms list as a requirement, or tell people we're compatible (with all the ambiguity that brings), or lie, and say we use MySQL even if we don't. Of those choices, I chose the former.
Jeff