Why are modern regional jet designs shifting towards under wing engines?
The underwing engine location allows for a lighter airframe.

Placing the engines under/ahead of the wings brings these benefits:


Bending relief: Engine mass is closer to where lift is created, so the structure has less stress to carry around.
Flutter suppression: Placing the engines ahead of the elastic axis of the wing shifts the flutter speed up, so the wing can be built less stiff.
Better evacuation: With wing-mounted engines a rear door is possible to create a second point for emergency egress.
Lower noise level in the rear cabin.
More useable cabin space at the rear.
Easier access to the engines for maintenance.
Shorter piping for fuel and bleed air. Especially in the narrow fuselages of regional jets the air ducts for ventilation take a lot of valuable space.
And don't discount psychology: The underwing engine design looks more like the bigger jets, which can be a big factor for some passengers. The airplane looks safer and more mature.


With modern CFD it is much easier to reduce the interference between the engine nacelles and the wing, so the biggest disadvantage of an underwing engine location can be reduced.

Note that early designs for the Boeing 737 used rear-mounted engines. By relocating them under the wings, the structure could be made 700 kg lighter. Quote from www.b737.co.uk:


  Overall, the wing-mounted layout had a weight saving of 700Kgs over
  the equivalent “T-tail” design and had performance advantages.


The rear-mounted engines were a fad from the late Fifties (started by the Sud Aviation Caravelle) to the mid-sixties and driven by the concern about the higher risk of low-mounted engines ingesting debris and the higher asymmetry in engine-out conditions. With more operational experience and more reliable engines, these concerns proved unfounded. Citing www.b737.co.uk again:


  Initial worries about the low mounted engines ingesting debris proved
  unfounded, this was demonstrated by the Boeing 720B whose inboard
  engines are lower than the 737's and had been in service for four
  years without significant problems.


Small business jets still prefer rear-mounted engines so they can operate from more airfields (on which the runway might not be as clean as those of big airports) and because the weight penalty of rear-mounted engines is less pronounced. Instead of bending and thrust loads, handling loads and manufacturability drive the minimum skin gauges on smaller jets.