As he said, 'more than Emacs is at stake here.'.[1]
I presume this refers to Bazaar's status as part of the GNU project, and that RMS did not want to write off part of GNU without being certain he needed to.
Regardless, he has since OKd the switch from bzr:
I don't insist that Emacs should stay with bzr. I chose to support bzr because it was still a contender at the time. [2]
I mean, don't not fixing what ain't broke is certainly a matter of integrity. Fixing something to the latest hippest new fad just to "attract young hackers" detracts from integrity. Making the editor an awesome badass attractive editor should be enough. Otherwise you'll only be attracting groupthink-prone douchebags, anyway.