He difficulty BE have with the E175 is that it doesn't hold many more passengers than the Q400s, isn't a whole lot faster due to the short flight times and relative speed of the Q400. On the other hand it burns more fuel and has expensive finance and maintenance costs - I believe the engines in particular. Customers may have a preference for jets, but they won't pay more. The US is slightly different as stages are longer (probably too long for a prop) and mainline union contracts prohibit regional airlines operating aircraft with more than 70-78 seats. Tellingly the E190/195 hasn't been a big seller in the US for exactly this reason - across the airlines cheaper to "misuse" an A320/737 on a few routes.
Delta seem to think the C-Series is a different argument - even at mainline pay. It's a lot bigger than E175 and a bit bigger than the E195, but if the C-Series is to short-haul what the 787 is to long haul flyBe could carve out a niche for themselves if/when they are in a position to grow.