My back bets keep losing, why would a bookie gub me?

Bookies aren't interested in your short-term profit or loss. They care about the long-term.

When you are matched betting, the expected value is in your favour. This means that, even if you are losing at the bookie right now, you will eventually win over time due to the fact that you are placing +EV bets.

It is therefore in their interest to shut you down as early as possible.

They opposite situation to this would be the roulette table in a casino. 

In roulette, the expected value is in the favour of the casino (and your bets are -EV). So even if you go on a winning streak and are up thousands, they are always happy to let you keep placing bets* because they know that, in the long run, they will get it back.

(*unless they think you are cheating of course)