The motivation here is to to provide a smoother experience for first-time users of Discourse, so that they get a more “fully featured” Discourse experience right out-the-box.
Another motivation is the developer experience for our team, and for community contributers. By having all of these plugins bundled with core, there’s no longer a need to consider compatibility with different core versions. That’s particularly useful for plugins like discourse-ai which are being very heavily developed right now, in tandem with related core changes.
The branded Auth plugins are the ones which are most likely to be absorbed into core, just like our existing core auth methods like Google/Facebook/etc. So there’s a good chance they’ll be removed from the list in the not-too-distant future.
This change does not relate to performance on our hosting. We already had a pre-built distribution with all of these plugins, and more, as you describe. 
I’ve fixed the ordering 