When I was using the mobile version of an SE site I got prompted with the error:
new tags cannot be created from the mobile site. The tag 'sometag' is new.
Why is this so?
Probably because it's really easy to make a typo when using a mobile browser, and they don't want to clutter the tag-space with misspellings, so it's prompting you to recheck.
For legitimate cases, just edit the tag in later when you're not using a mobile browser or switch to the full-site version.
I've read waiwai's answer, but I still think the restriction for creating tags on mobile feels a bit arbitrary.
If a user has the required rep to create a tag, I don't think their choice of platform should act as a limitation.