Easyjet do have A320's.
A319's are easier to fill than A320's
319's cost less than 320's to purchase and due to the fact that they are lighter they burn less fuel for the same air distance.
There's no point in buying a 180 seat 320 if you are going to struggle to fill it. Far better to get a cheaper 319 and fill it up every trip.
I'm not sure how many seats the Germanwings 319's have but EZY messed up by putting too many seats in their 319's. If they'd of stuck with 150 seats they would also have been able to make huge savings as you only require 3 cabin crew on 150 seat aircraft. They've got a procedure for reducing the number of seats but it seems to cause more trouble than it's worth.
SW