The marriage certificate isn't enough proof because it could be a sham marriage purely for immigration purposes. And length of marriage alone doesn't matter either, you need to prove a genuine and ongoing relationship for as long as you have been together.
Originally, for a fiance visa, you needed to show proof of relationship and communication, but all you needed to show for a spousal visa was the marriage certificate and a photo together.
However, they changed the rules a few years ago, and now you have to show evidence of the entire relationship (communication and/or living together) for a spousal visa as well.