Easyjet route planners will not think "what unserved routes can we open at an airport to give pax the maximum choice of holiday destinations". Instead they think "how can we deploy aircraft to get the maximum profit"
Easyjet know Birmingham is not a strategically important airport for Virgin long-haul, and Flybe/Connect are perhaps unlikely to defend their BHX routes heavily if facing strong competition from a well-funded rival.
Or to put it a more crude way - Easyjet can smell blood in the water at Birmingham