Why don’t banks explain why they freeze or close accounts?

Predominately three reasons:

1. They are not allowed by law to disclose. It could be something serious/not-so-serious, but it needs to be reported or investigated, they don’t need to tell you that.
2. Any explanation they give will start a chain of counter-clarifications or challenges from your end. They just don’t want to get into I-said-they-said argument.
3. They have been ordered to do so by the financial regulator.

The best thing they can do is to freeze the account (so that no more transactions can happen on this account). If they close your account it means, it is something they don’t want to associate themselves with you, for economic and legal reasons.