Probably because old stock and RIM feels the end user won't be lazy enough not to be able to update the os themselves. I could be wrong regarding the old stock though.
AT&T still only provides and supports OS 5 officially. They have yet to even offer OS 6.
Now we know why AT&T never mentioned OS 6 when they originally announced the Pearl 3G a year ago (even though RIM said all along that the 9100 would support it). Yet the Curve 3G was advertized as OS 6 compatible. Sure enough, the AT&T Curve 9300 eventually got an official carrier approved OS 6 update. Not so for the AT&T 9100.
Posted from my CrackBerry at wapforums.crackberry.com
got my pearl from my carrier with OS6 and unlocked just like my bold from another carrier and unlocked also makes dumping the old carrier in favor of the newer one much easier.
i have the pearl 9105 (uk version) only vodaphone uk has released os6 OFFICIALLY!
the problem is os6 os the pearl creates a masive memorie leak
i updated it by deleteing the vendor.xml (im on 02-uk) and i deleted social feeds
and all the other stuff i did not use,and like 1hr of browser use it went from
70'ish MB to 30MB no apps or internet would work,only got 4 apps (appworld,bbm,foursquare,facebook) but i dont use the app for facebook
i just dont think the phone is powerfull enough to run os6,and i think RIM/Carriers know that
and i think thats why they realease them on os5 cos it runs better
just like my bold from another carrier and unlocked also 