I don't know about generally, but in the UK, Denmark and the Netherlands the husband of the queen is called prince consort (though in the Netherlands the queen has recently abdicated in favour of her son).*

In the Netherlands the head of state is, in the constitution, referred to as King, even if she happens to be queen. Calling the queen's husband king would be confusing.

Some countries, like France, followed Salic Law, which does not allow women to inherit land. In those countries the issue of what the queen's husband should be called obviously did not arise.

• This is outside the 20 year rule, but i feel i should mention that there was some debate in the Dutch parliament, as to whether the wife of the current king should be called queen, or, for the sake of equality between the sexes, princess consort. In the end in was decided she should be queen.