I don't know Gaelic, but I do know a little about spelling!
If a language gets written down, the spelling becomes standardised. Then, if the pronunciation changes later, the spelling no longer matches it. That's Point #1. Example being the cuckoo - in Middle English the word cuckoo copies the bird's call exactly, with the u being the same sound as the u in up. But then the pronunciation of English got out of step with the spelling, so now the English call the bird a coocoo while it goes on happily speaking Middle English.
Point #2 has to do with colonialism. In South Africa
, two of the local languages that are very closely related, closer than Spanish and Portuguese for instance, and mutually comprehensible, have different spelling systems because they were originally written down by missionaries who spoke different (Western) languages. So one set of spellings looks English and the other, French.
Point #3 is related to Point #2 - even though it might make life easier for people who don't speak your personal language, say Gaelic, you might well be reluctant to change the spelling to make it easier for them. Who are they, anyway? Historically, the oppressors of your people, very often. (A friend of mine worked in Dublin for some years and developed a rule for Irish spelling: In order to spell any Irish word, take the alphabet. Throw it against the wall. Look at the letters which end up on top, and remove those which make or seem to make recognisable English sounds in the word you want. Repeat until you have a set of letters with no visible connection with the sound of your word. That is the correct Irish spelling ...)