Why is 146 cheaper to lease than Q400? More of them lying around to lease is why - Ansett going down, Aer Lingus chopping theirs etc. It's an older aircraft and thus has most of its capital value written off.
It is reduced to specialist hot/high roles or for places like LCY, while 717s start replacing them in mainstream use.
As for Q400 orders vs CRJ - when Air Canada starts buying EMBs as well as CRJs rather than flying the flag and giving Bombardier the entire RJ order it makes you wonder, plus Bombardier shares are depressed by the fact that a lot of outstanding CRJ orders are with US airlines in/near Chapt.11.
With NIMBYism rampant, runway extensions for RJs are not going to be easy to come by, and Q400s can get away with about 4600ft field length as opposed to 6,000 for CRJ and more for the smaller EMBs.