A couple reasons ~

Your neighbor's pool has no fish. Tree frogs are small and
are wary of getting into a pond with fish as they might get
eaten. I had two ponds right next to each other. One with
fish and one without fish. They all flocked to the fishless
pond.

Greenfrogs will hang out in a pond during breeding time but
do not always stay around after breeding. Bullfrogs are the
most aquatic of frogs and love to hang out in the water.
The bigger they get the more likely they are to eat fish.
A good population control measure but usually it is your
favorite fish of the current babies who gets eaten!!

Frogs will often come back to the pond where they were
raised to breed. If all the eggs and tadpoles are eaten by
the resident fish then there is nobody to come back to the
pond.

hope that helps,
k :-)
http://tinyurl.com/6bguh ~ new pond keeper info
http://tinyurl.com/yp64db ~ slide show of pond