A few things come immediately to mind:
1. It requires an extra pump & plumbing to move water from the gray to the toilet bowl.
2. Gray water is really stinky and scummy, so many people won't want to use it anyway
3. The fresh water flush source would still be needed as an alternative when there wasn't enough gray available. To do it properly, the toilet should really have two flush inlets.
I've heard of people who draw off the gray water in a bucket and use it to flush, usually while boondocking for extended periods. Give it a try.