It's really very simple.
Most of the time HTTP is used to fetch web pages or to upload form data.
Printing is viewed by IESG to be sufficiently different than either
of these two that it needs to be distinguished from ordinary HTTP, both by
having a different port # and a different URL type.
Making such judgements is part of IESG's job.
I refer you to RFC 2026, section 6, first paragraph:
6. THE INTERNET STANDARDS PROCESS
The mechanics of the Internet Standards Process involve decisions of
the IESG concerning the elevation of a specification onto the
standards track or the movement of a standards-track specification
from one maturity level to another. Although a number of reasonably
objective criteria (described below and in section 4) are available
to guide the IESG in making a decision to move a specification onto,
along, or off the standards track, there is no algorithmic guarantee
of elevation to or progression along the standards track for any
specification. The experienced collective judgment of the IESG
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concerning the technical quality of a specification proposed for
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elevation to or advancement in the standards track is an essential
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component of the decision-making process.
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Keith