Abbreviation is a tool that allows writers to condense a phrase without losing meaning; the letters removed are unecessarily redundant because the information can be transmitted equally well without them.
This relies on the writer and the reader/listener having an equivalent understanding of what the abbreviation means. For the simplest example of how easily this can go wrong, ask yourself: could you run a distance of 100 m in a day? Your answer is very different depending on whether you understand the unit m to be metres or miles.
Stack Exchange supports a global community, to its great benefit. This means there is a responsibility to serve all of the community equally. Online communities frequently devolve to a USA-centrism, even when this is not justified, and tags with abbreviations is one more area that this bias could easily slip through, to the detriment of inclusion and understanding for everybody.
Therefore, if an abbreviation is to be used, it must pass those two tests:
It is a common and established abbreviation for that group, event, action or idea for the people who use it; and
It is not a common or established abbreviation for a different group, event, action or idea for other people using it notably elsewhere.
Failure on either point means the abbreviation is not common enough to be understood alone, making it pointless, or that it cannot be fairly used just for a single one of its multiple possible meanings, and the full phrase or a non-abbreviation short form must be used, as appropriate to the case and respecting limits of tag length.