Two possible reasons I can think of :
1 Cost
2 Maybe it was felt that, given the sensitivities of this visit, a british military a/c landing at an Irish base on a non-emergency basis might have been felt inappropriate.
Contrary to press reports, it is not just a few hotheads who are opposed to this visit, although only those few will take direct action. A large part of the population feel that the country cannot afford the €30m+ this and the O'bama jaunt is costing us, with no real pay back apart from journos hotel bar bills (large though these are likely to be); and a significant number are not overly happy with the situation in the six counties even today although they are opposed to a violent resolution.
My thought was how could a one-aircraft company run by an Indian businessman in Birmingham garner such a high profile contract.....
MD