Because they're structured differently. A man page for an app is just that, a page, with some formatting and sections. Texinfo is more like your typical website - lots of small cross-linked pages with a common index. Consequently, a man page is usually an extended take on --help, while info pages are more like product manuals.

This is the theory. The practice is that GNU mandates Texinfo for its projects, and because documentation tends to be the weak spot for all open source projects, manpages end up the most neglected as a result, which can be quite annoying. Especially since pretty much nobody else uses Texinfo - man pages are good enough for most console apps, and those that need more detailed documentation use Docbook, Markdown etc.

Historical difference in style and preferences. GNU’s Not UNIX and all that Jazz, so they have different conventions, some of which would not translate to a man page that well. Plus Texinfo is a tool for generating multiple output formats from a single source.

Typically (unless this changed at some point) the info pages are the canonical reference for GNU projects and the man pages are to accommodate Unix hackers.[1]

[1] https://www.gnu.org/prep/standards/standards.html#Man-Pages