Commercially it has become accepted wisdom in the LCC world not to mix your types in the fleet if possible, and certainly not to mix them at a single base. Easy have made the transition from 737 to Airbus base-by-base, and the two types are of similar size, therefore avoiding many of the pitfalls (though not all).
The additional size of the A321 in Easy configuration (what, about 220 seats ?) would compromise the standard turnround time allowance and extra time would need to be allowed for this, impacting on daily productivity. It's a difficult size for a Low Cost operator.