Cornish Jack, I think you hit the nail on the head. The main reason is because there is no need to have it pressurised. Like all systems on this generation of aircrat...they are complicated. The aircraft is reauired to have an alternate extension system for certification purposes. The cheapest and most reliable alternate gear extension system is a freefall system. In order to have a freefall system you must have uplocks which can be released by a handle (or switch for the more modern boeings). So...if you have an uplock why bother to pressurise the wntire retraction system in the cruise. If it springs a leak you could dump your whole hydraulic sytsem fluid overboard in order to keep the gear in the up position...when there are uplocks to do the job. It simply makes no sense to pressurise the gear retraction system for a variety of reasons when you have a mechanical uplock system.