Can anyone tell me the rational of upgrading to SAAB then CRJ, but not CRJ to CRJ?

Example: A SAAB F/O could upgrade to SAAB captain, then to CRJ captain. But, the same is not true for CRJ F/O (CRJ F/O can move to be SAAB captain then CRJ captain) is this really making sense to anybody?

Why could they just allow F/O to upgrade from their respective equipments? If you've been flying as an F/O on RJ, it makes sense to move to the left seat on the same equipement!

And really, how long does it take to upgrade from either equipment?