The use of a 'proper' aircraft will also entice leisure travellers.
Yes it's a nice big comfy A3xx but a 65nm route is really not an efficient use of the type. It will be barely topping-out at FL150 before commencing descent.
Unless they operated IoM as a pass-through onto other destinations, as JEA used to with the F27s, I don't see how this will be much more affordable than Eastern.
To put it in context, the only shorter routes on an A320 that I can think of are Fuertaventura to Arrecife which are really positioning flights with pax permitted.