The main reason is that it often costs more money to build a
fluorescent-based system, than it does to pick up a used mercury-vapor light.
I am using a nuArc 26-1K which safely houses the bulb, (yes, in a
reflector so I can print up to 24" square) and it came with an electronic
integrating timer and a vacuum bed. I got mine for $350, but I've seen
them for less.
Also, the mercury lamp is more of a point source.
dave