This has baffled me, too, as long as I've had a pond. The first year, I had a few frogs. Then the newts arrived (and yes, one of them famously did get p!ssed, on the beer in a slug trap. I put him under a cold shower and he recovered - slowly)
I haven't seen a frog since. But I have the national collection of palmate newts.
I gather, as the Hopeful Herpetologist says, that newts eat frog spawn. In that case, how can they ever co-exist in the same pond? And given the muscular attitude of the newts, how do any frogs survive..?
But they clearly do. So now I know.