There are lots of little answers to that question. For instance brew is a third party addition to OS X whereas its equivalents in Linux-land are integrated into the main OS. Brew does a surprisingly good job, but it still has to work around the fact that the main OS wasn't installed with brew.
But there are two big meta-reasons:
The first one is that since much of Open Source in general is done to "scratch ones on itch", Linux can be seen to be "by developers, for developers". This is generally seen as a negative for non-developers, but for developers it is a positive.
The second one is that if you're a web developer, your deployment platform is going to be very similar to your development platform. This means that you only have to deal with half the number of platform specific bugs and configuration. More importantly, you discover these platform specific bugs and configuration issues sooner.