Internal polls are more accurate because campaigns use them to decide where they are going to spend their money. If you can’t get accurate polling you might be wasting money where it could be better spent elsewhere.
Bingo. When you're spending a few hundred million dollars, you're going to spend millions on polling to make sure you're not wasting the hundreds of millions. That's a lot more than the "regular" polling firms spend because there's so much at stake for the candidates.
You can tell what's up with the internals by where the candidates are appearing and where they're buying ads.