Well, there is a whole ecosystem around RHEL, which RH probably wants to preserve. More frequent releases would demand faster certification from Oracle, IBM, SAP etc. Which is very unlikely. They mostly only certify new products. Customers would be unhappy.
Yearly releases would only work if RH would provide the whole stack, but for most people they don't. I think they want to get there, but so far ..
I think most customers are happy with RHEL5 and are only willing to switch for compelling features. So if a 2 year update cyle won't offer those RH will adopt a longer cycle, which is what has happened I guess.
IMO this is RH new release policy:
"Gather enough features that would compel our customers to switch and that we can't realistically backport and then release.(and adapt lifetime of products accordingly. 8 or 9 years max)"
Maybe that is the crux of the subscription model, you have to do what your customers want (I know how strange that sounds.)