Because all the projects currently built were built under those rules. Those who built them (and their investors) did so based on the rules. Changing the rules retrospectively will make it difficult for new projects to get investment as there is no certainty. The rules could be changes for new generation projects going forwards though
The investors were expecting a return, which is fair, but not 300% in 12 months.
It's clear the system itself is needlessly making the cost and inflation higher.