The quote you reference in the question is as follows:
Doctor: We're not meant to be here. The Tardis draws it's power from the universe, but it's the wrong universe. It's like diesel in a petrol engine.
Mickey: But I've seen it in comics. People go hopping from one alternative world to another. It's easy.
Doctor: Not in the real world. It 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, and took it all with them. The walls of reality closed, the worlds were sealed. Everything became that bit less kind.
S02E05 - Rise of the Cybermen (Number 10)
Once the Time Lords came back, they were put at the end of the universe.
General: Gallifrey is currently positioned at the extreme end of the time continuum, for its own protection. We're at the end of the universe, give or take a star system.
Doctor: I know. I came the long way round.
S09E12 - Hell Bent (Number 12)
The important thing to note is that when the Time Lords kept their eye on everything you could hop between realities. However, since the Time Lords are now at the end of the universe, they are not keeping an eye on things, and thus unable to regulate (and enable) hopping between realities.