[LoSboccacc]

two major reasons:

reach: get enough upvotes in Reddit and you can attract a lot of people including prominent figures in the space to look at your build

friction: people passing by might not want to create a profile just to comment on your build, but if they stumble on the build on a site where they already have registered it's so much more likely for them to speak their mind

a mass of users, even from unrelated topics, carry non linear value in social interaction based sites, and so it happens grouping topic together wins over fragmenting over multiple small sites