Many countries do exactly that, sometimes with a few exceptions (ex: expats, disabilities, ...).
One problem with internet voting that does not apply to money is the "receipt-free" aspect. That is, a voter should not be able to prove that he voted for a particular candidate, as it would allow for vote buying, threats, etc... And it is a hard problem. With money transactions, you generally want the opposite, which is an easier problem.
* It does apply to most other internet systems.
* Things like banking fraud can be detected and remedied. Election fraud is much harder to detect and even harder to remedy.
* Voting requires anonymity. Most internet systems are not anonymous: you are identified by your IP address at the very least.