Is there a reason GaN chargers stop at 65W?
If the input power of the adapter is less than 75W, you don't need a power factor correction stage (by EU and other regulations).  

So they are easier to design, and they hit the largest segment of the market.  

Once you need a 2 stage converter you have additional components and thermal considerations and GAN probably doesn't make as big a performance and size difference as it does with a single stage converter.

There are single stage PFC converters, but they suffer from various performance issues.