Is it possible to pay for iCloud yearly instead of paying a monthly subscription?

As stated, no, and actually Apple does not say why.

If you have an annual iCloud storage plan - Apple Support

There are now no longer any annual plans, only monthly unless you have a very old plan that is still set to annual payment. If you do not want to add a credit card to your payment information you can use a gift card to make sure your iTunes Store balance has enough money to make payments automatically over a long term.

No.

It would cause to many rebates for Apple.