The company (Symbolics) was owned by Andrew Topping. Topping died, and Symbolics' IP was sold at his deceased estate auction. It was bought by John C. Mallery, a (former?) MIT professor, so he is the current copyright holder. (Or at least he asserts he is–some have expressed doubts over the legitimacy of the transaction, but the only way to get a definitive answer to that would be through a lawsuit, and nobody thus far has wanted to go down that path–the risks are that one might spend a fortune on lawyers and Mallery could win.)
Gary Palter was one of the last employees, and as such doesn't own the copyrights. However, he is in personal contact with Mallery, and the enhancements he has made to Genera (such as porting it to ARM) have been done with Mallery's permission.
Nobody seems to understand why Mallery is squatting on this rather than making it publicly available. Palter has never clearly explained it, although I imagine he doesn't want to burn his bridges with Mallery, and that may limit what he's able to publicly say.