Lack of capabilities at these airports is part of the reason. They all have short runways and no (or few) jetties.

There is also loads of space at Manchester, so why divide your market?

Also, for short hauls you can rely on point to point traffic. For long hauls, you need connecting traffic as well. LHR is very good for that and to a lesser extent so is MAN. The other airports don’t even begin to be competitive. Look at MAN’s long haul flight; only a few are point to point. Most offer connections, which is why they can be viable. Emirates, Singapore, US Airways…