Newts and frogs are not mutually exclusive but they do tend to have a bit of a boom-bust relationship.
Newts eat tadpoles, so ponds with lots of newts tend to have fewer frogs. But then a decrease in frogs means fewer tadpoles and so less food available to newts, which can lead to fewer newts and so more frogs in following years - and so on.
It may be that a pond is just more suitable for newts than frogs. This isn’t a bad thing: newt habitat is just as valuable as frog habitat and newts are equally interesting to watch.
We have three species of newt in Sussex: the Smooth Newt, the Palmate Newt and the much larger Great Crested Newt.