Because it's a lot of effort for a relatively small gain, basically.
In order to have cross-platform gaming you have to ensure that there are no differences in game physics, level design and so forth and that patches are issued for all platforms at the same time (or otherwise you break cross-platform, which happens a lot on games that have done it) so that these things are kept in sync.
Having achieved this synchronisation you then need to ensure that one of the platforms doesn't have an advantage due to control issues, etc. or people will whine that it's not fair.
In order to have cross-platform gaming you have to ensure that there are no differences in game physics, level design and so forth and that patches are issued for all platforms at the same time (or otherwise you break cross-platform, which happens a lot on games that have done it) so that these things are kept in sync.
Having achieved this synchronisation you then need to ensure that one of the platforms doesn't have an advantage due to control issues, etc. or people will whine that it's not fair.