And I'm still yet to see a Jet2 aircraft at Humberside...
The only reason Ryanair are operating this flight is because a large proportion of the seats have already been bought by tour operators, so that's the majority of the operators costs paid for if indeed it doesn't already put the route into profit.
The remainder will be sold to the general public, which will be the icing on the cake for Ryanair.
You haven't seen Jet2 into HUY for the same reason that you don't see them at many other similar airports:
1) they are already operating very profitable routes elsewhere,
2) they don't have the same aircraft numbers as Ryanair to bid for extra summer work and,
3) almost certainly, their operating costs are higher so they can't be as competetive as non UK based operators when bidding for extra summer work.