Because Uniden must pay for licensing agreements to use the technology, and not all users want or need the upgraded modes, so instead of charging all users an extra $120 + for the upgrades the offer them al a carte.
And, before you compare them to Whistler Scanners which do not charge for the upgrade remember, Uniden scanners do true trunk tracking on the systems while the Whistlers just scan each of the frequencies for traffic and do not do true trunk tracking.
This was asked and answered many times over when the paid upgrades first became available, a search of the forums will yield many posts on the subject.
In a nutshell, Uniden has to pay ongoing royalties and licensing fees on each scanner that has the upgrades enabled. Uniden chose to not charge every customer for the upgrades by raising the cost of the scanners across the board.
You will find many posts by using the search feature in the top right, as @fxdscon suggested, the suggested similiar posts when you create new threads do not include all posts.
I'm not saying that. I'm pointing out that Whistler isn't licensing DMR. Perhaps the IP lawyers will chime in. I guessing they are getting around the licensing by using DMR in a conventional system, programmed as a trunking system.