Ask HN: What are downsides of multiport memories?

Why we don't see them used more widely outside of extension cards like graphic cards? Are they much more expensive? Are they slower/have higher latency? Do they consume much more electricity? Or they simply are not the answer to at least part of problems created by multi CPU computing, so no one uses them as "main" RAM. Or there are some other subtle reasons? Do they create new problems for operating system design when used for main computer memory?