I like to think that in order to feed he has to do something similar to battle meditation and is thus susceptible to physical harm whilst doing so. He can't just pull that kinda power out at a whim.
When the Exile managed to get to the bridge he simply didn't have the time to do it, or the strength, as he was coming purposefully to Telos to feed in order to get the strength to carry on and find more. It's like an endless addiction that slowly drains you when you're not feeding it.