"Many people had contributed their efforts to the Redis project for free - both in terms of code but also in advocacy, writing tutorials, publishing example code etc"

I can understand that, but the thing about the BSD license is that such value never gets lost. People are able to fork, and after a fork for the original project to still lead will be require to put something more on the table. Inside Redis it was tried very hard to don't change license. For years it was some kind of "dogma", something one could not even speak about. Then after all the kind of attempts, and following the experiences of MongoDB and others, finally this choice was made. (Disclaimer: I saw the switch as an outsider, so my information is not complete about the final decisions -- I for sure know how this was always considered to avoid if other setups were possible).

However now I see that there is the case for giving the community something in exchange to the license change: a lot of good things in the core, a very good attitude towards the community, and so forth.