E190 capital costs are not half of 737, in fact they're quite close, per seat they are higher. Fewer lessors deal with regional aircraft, and that reduction in competition adds further to cost structures. Boeing and Airbus offer deep discounts with vastly higher production rates and economies of scale that EMB and Bombardier can only dream of (unless/until they fall under Boeing/Airbus umbrella).
The engine maintenance costs of the E-Jet series has often been publicly cited as a problem by Jetblue, VA and others. The E2 may be a serious weapon in the near future, sharing engine technology with the CSeries and A320neo.