Why do some US banks restrict or block international wire transfers?

Predominantly risk exposure. Person-to-person (P2P) transfers are seen as high-risk transactions for most banks (albeit this is not the case). To suppress that risk, they refuse to provide international bank transfers.

In the US, however, most small banks do not have the facility to do international transfers. They are not members of SWIFT and also do not have an intermediate relationship with the FX banks (mostly in NY) to aid international transfers.

Not to mention business volume. A bank may not find it suitable to invest large sums of money in technology, compliance, monitoring, and reporting for just a few hundred international wire transfers a year.