Why does Enceladus have subsurface pockets of water/oceans
when it shouldn't?
The Saturnian moon's oceans should have frozen solid in less than 30 million years.

Tidal flexing doesn't explain it; it doesn't produce enough heat energy to
keep the Enceladus' oceans liquid.

There are not enough radionucleides inside the moon to keep its oceans liquid.
So what's the story?

I could draw some anomalous conclusions, claiming that Enceladus the moon
is younger than 30 million years; but if that's the case, how and why would it form so late
in the Solar System's history?

I could claim that the moon of Saturn may have suffered a statistically anomalous
recent history comet impact [in the last 30 million years], resulting in reheating
of its subsurface ocean, keeping it from freezing entirely...But when you think about it
it doesn't make sense...
For example: if a comet impact reheated the freezing ocean beneath Enceladus
were's the crater on the moon for such a late history impact?

Enceladus is a mystery wrapped in an enigma wrapped in a puzzle.