Thankfully I am not at Lucent anymore and am not privy to the tortured negotiations that ended up at the obviously inelegant compromise of "The University of California, Berkeley, has been authorised by Alcatel-Lucent to release all Plan 9 software previously governed by the Lucent Public License, Version 1.02 under the GNU General Public License, Version 2." But the odds are overwhelming that the one-word answer is "lawyers".
AT&T gave away Unix during Unix's formative years, because AT&T's government-granted monopoly disallowed it from making money on computers. They didn't make any money on Unix then. They tried later, and made some, but what a sense of loss about what might have been if only they'd been charging from the beginning! Of course, if they'd been charging from the beginning, it's hard to say Unix would have been so popular.