Why don't short-haul regional jets go for composites?
The C-Series has a carbon wing box.  The fuselage is aluminum but is a newer fairly exotic aluminum lithium alloy that is a bit lighter than 2024.  The decision to go with aluminum for the fuselage was mostly development and manufacturing cost (a fuselage requires a massive autoclave and it's development is fairly high risk the first time around) and ease of repair (the fuselage takes the brunt of vehicle damage on the ramp, especially on smaller aircraft) compared to carbon.

Cost is probably the number one driver.  They stuck with things like bleed air services, and wisely decided to avoid lithium ion batteries (Boeing had to use L-ion with the level of electric services).  The C-Series is overall a more conservative design than the 787, and it still almost broke the company, which was forced to give the airplane to Airbus for free or go under.  

The E-jet is more or less the old 190 with a complete systems state of the art revamp (and I believe a new airfoil) that make it more or less a smaller C-series system wise without the risks of composites development.  An even cheaper choice, and a wise one on Embraer's part.