The whole Desilu/Paramount/CBS process has actually become something of a closed loop that started way back with the genesis of I Love Lucy:
1. 1951: Desilu is the name that Desi Arnaz and Lucille Ball give to their little production company that CBS contracts with to produce the new show they are creating. CBS, as per the norm, will own the show's rights - which, since the rerun does not exist in 1951, are not worth all that much. The only fly in the ointment - the Arnazes do not want to go to New York to do the show live, with a kinescope for the west coast. They want to stay in California. CBS balks at this, saying it will cost too much - especially since they, especially Desi, want to film the show in front of an audience, as Lucy NEEDS an audience to be funny. Since this was never really done before, CBS says it is too expensive. Desi then offers that Desilu will pick up the extra cost - if rhey get to own the negatives of the show, CBS, figuring they are making out on the deal agrees.
2. c.1953/4: I Lucy, as we all know is a HUGE hit - the biggest in TV so far, and Desilu is deluged with other stars wanting to do shows 'the Desilu Way'. Desilu needs a bigger facility than the little two bit facility they are leasing from, so Desi cuts a deal with CBS where they will buy back the films from him, so he can buy a small indy studio facility named Motion Picture Center. Couple of years later Desilu is so successful, Arnaz is able to finagle the purchase of the old RKO Studios - which are separated from the Paramount Studios literally by a brick wall. This purchase also includes the famous 40 Acres backlot. Desilu is able to then the center of shows produced by independent producers in Hollywood.
When it comes time for Lucy to find a buyer for her struggling studio in 1967, it is almost (excuse the expression) logical that Gulf & Western, who already owns the lot next door, be the purchaser.
So basically, when the Trek properties were bought by CBS, they basically were reuniting with their 'Lucy' ancestors!