I think the CRJ (& probably ERJ) were built with very short lifespans in mind due to the fact they were designed for short routes & quick turn arounds (especially some of the very short routes in the USA). Due to it making lots of short hops, it would reach its maximum airframe life very quickly.
I'm sure there are some CRJ's which are used on the short Commuter hops in the US with the same amount of airframe hours as some of the older B747's!!
Fried Chicken