I think the thinking behind dropping the support is something like this:

I use browserify/webpack/node and npm is great. I wish more libs that I use were also on npm so that I wouldnt have to fork/shim them. How can I force other people to use npm? Maybe if I drop bower support for my awesome lib that everyone uses, other libs will follow suit, or perhaps the users will see the light and stop using bower because it sucks.