Why do small jets usually have one low flap setting?
The provision of lots of flaps settings is motivated in part by the desire to provide as many profile optimizations as possible. Generally, the less the flap extension for takeoff, the more efficiency you have in a departure plus better engine failure performance, but you need the longest runway, so you would use the least extension allowable for a given runway.

The CRJ was originally certified without an 8 degree flaps setting, and it was added as an option a year or so into production.  In general, you will always use Flaps 8, unless conditions require Flaps 20 (shorter runway).  

It can create a trap for the CRJ because if you are departing an airport where 20 is required to meet departure performace requirements, and you set Flaps 8 by mistake, there is no Takeoff Configuration Warning mode to warn you you're at the wrong takeoff flap setting (only a warning for 0, 30 and 45... it can't know which 8/20 TO setting you actually want) and you can take off with insufficient flap extension and get into a bit of trouble when you rotate at the Flaps 20 Vr and you don't lift off.  When departing a short field that required Flaps 20, you had to be extra careful to make sure Flaps 20 was selected.