I would say there are just so many unknown variables (probably impossible to know), to the extent that people can get away with pretending that it's non-deterministic, when it actually is deterministic.
Which is why there's no way to be certain that it is or is not deterministic. According to current knowledge, quantum uncertainty is not deterministic -- solving the Schroedinger wavefunction for hydrogen, for example, shows no way to predict the future location of an electron. The best we can do is a probability distribution.
If, however, as the parent mentions, hidden variables do exist behind quantum mechanics, it could turn out that it was deterministic all along.
> "Equation X doesn't help us understand phenomena Y"
How does that even remotely suggest non-determinism?
> If, however, as the parent mentions, hidden variables do exist behind quantum mechanics,
Isn't it obvious that we're not even remotely close to knowing everything?
The uncertainty principles simply suggests that we perhaps can never grasp or understand all the details; it doesn't in any way imply that these unknown details don't exist. It'd be foolish to think that "because we can't predict it therefore it's non-deterministic".
