I'm surprised that Microsoft chose to go with an ecosystem that is as volatile as nodejs. By the time you read this post npm, gulp and bower will be obsolete. Grunt is already ancient news.
Both npm and bower seem to work OK in .NET Core. My only issue is the amount of data they dump into your project. The packages take up a lot of space so they take a while to restore and you need to be careful not to add them to source control. For example, by default the node_modules folder takes 18MB of a 20MB project and has over 3000 files in.
Yes, if you focus your vision on only the new and hot tools. NPM is still the package manager for node.js and that's not going to change for the foreseeable future. Grunt is still in use by lots of people and companies and definitely has its uses, but might not be everyone's first choice for a new project. Enough with the hyperbole.
I don't think there was any choice, the forefront of Web Development is on node.js/npm asset pipeline - if they chose any other solution they'd just be forever playing catchup.
