XML isn't as common as one would think. It's been steadily decreasing in popularity and use. It's a very verbose format that is suited to enrich a larger set of data, such as HTML documents. For data heavy documents where, it's a particularly bad match, as you end up using as much text for annotation as the data itself.

Using XML for 3MF is IMHO a technical cop-out, where you don't really want to solve it "correctly", so you go with something that is "good enough". With XML, you know it'll be able to encode anything, be human readable, and have existing parsing libraries in pretty much any programming language and standard libraries. So, it makes sense. However, if you're creating such a format, the least one should do, is write a sibling standard for how to directly binary encode the data. This isn't a hard thing to do. It just need a standard for how to do it, so everyone agrees. Here is an example online on how a rudimentary implementation could be done for OBJ files, but the principle is the same. That way you could chose to export either as 3MF or 3MFB (for binary), and as long as your slicer, and what not, can decode it, you're good.

The hard part of 3MF was all the great work in standardizing what, and how that is represented. The part where it writes that out in clear text, is the only part that is missing. They could probably just pick one of these too: https://en.wikipedia.org/wiki/Binary_XML