I'm sure there was a suggestion/rumour a couple of years ago, or maybe slightly less, that Easyjet could have 20 based a/c at MAN by 2020. Something like that anyway, (and not from the airline I might add) with an additional 1 or 2 based a/c each year. In 2013, it went from 6 based to 8 based, but remained at that this summer. Because there was no increase this year, I think some assumed that growth would resume next year. It may well still happen, but nevertheless a dangerous assumption to make.
Another factor is that EZY are I think expanding at LTN but not at STN, the MAG airport where Ryanair are reinstating frequencies and routes previously withdrawn. It may or may not be coincidence that it is RYR who are expanding at MAN this winter by continuing services started in the summer. Whether this reflects deals in place with the respective airlines, I've no idea, but to my mind it would be preferable to see steady growth by both carriers.
Perhaps another factor is that Ryanair seem more inclined to serve MAN with non-based a/c than are Easyjet. There is a significant proportion of RYR flights that don't originate here. Given the congestion early morning in the summer it would surely be beneficial if Easyjet could do likewise.