Minor spoilers for The End of Time Part 2...

My guess is either it didn't work out because of both Tom's and Martha's working schedules. Tom was always away in Africa while Martha was on assignmet in New York and other places.

Or, and I hope this wasn't the caser, Tom might have fallen victum to the Daleks when they had moved the Earth and tried to invade. He died protecting the children under his care.

Those are two theories I came up with. because Martha with Mickey was a total surprise to me.

Both of those antwoorden make a hell of a lot of sense!
Well, Adding on to either of those senarios, Martha is a LOT like Rose! (though the Doctor was to blinded door love to see it)(btw, not a Doctor/Rose of a Doctor/Martha fan) Mickey was originally with Rose and loved her dearly. The Only difference between the two is the fact that Martha decided not to travel with the Doctor anymore. Mickey wanted rose to make that decision the entire time He and Rose were together. He may have realised that Martha and Rose were so similar but Martha had the one thing that Rose didn't and gave her a try. It must have worked!