I will assume you are referring to new Doctor Who, since the Time Lords were only lost in the gap between old and new.

In the new Doctor Who they maintain that parallel worlds are now (roughly) impossible to reach.  That's why the Doctor is surprised to end up in one in episodes 5 and 6 of Season 2.  With the parallel universes being sealed off, Time Lords from other universes cannot venture into ours in the same way the Doctor cannot venture into theirs.

The relevant quote from the Rise of the Cybermen:

> Used to be easy. When the Time Lords kept their eye on everything, you
> could hop between realities, home in time for tea. Then they died,
> took it all with them. The walls of reality closed, the worlds were
> sealed. Everything became that bit less kind.

So the Time Lords being present allowed travel to parallel dimensions.  Once they were no longer there, it "locked" the universe, preventing Time Lords from other universes from accessing ours and vice versa.