If updates are rejected when they fail the signing check, then there isn't actually an advantage to HTTP other than one of principle (i.e. the view that all internet traffic should be encrypted). Update data can be compromised, but the result is simply a failed update and (presumably) a retry. The data which comprises a Sublime update is also not sensitive in any way – anyone can download it! – so it does not need to be protected.
Basically, there's no advantage to HTTPS in this case because of the signature check, and it is likely that sticking with plain HTTP simplifies the infrastructural questions associated with hosting.