Because, if they don't, the corrupt Democrats will say, "You did not do this through official channels so your candidates do not qualify for the upcoming ballot."
Don't believe me?
Courts: (before the election) "You can't challenge this election rule because you have not suffered harm."
Courts: (after the election) "You can't challenge this rule because the matter has already been decided by the election."
You're missing the point. Why would the Republicans, when organizing THEIR OWN convention, have to kowtow to the Commonwealth of Virginia's Department of Elections? What does a CONVENTION have to do with an actual ELECTION?
Sure, they're selecting their candydates. But that's it. That's not an election.
This is a classic example of the Repubs shooting themselves in the kneecap, if, in fact, they allowed this system to happen. State bureaucracies don't normally set themselves up without some kind of legislative action (meaning both parties). Though since Northam's been in office that may have changed.