Why the ‘o’ when ‘u’ would do?
It started about 1000 years ago - before printing existed - when scribes
(usually monks who were the few people who knew how to write)
decided that joined-up writing was difficult to read when you had several
downward strokes next to each other, as in the word
‘mum’ (mum). They were right: it was hard to
see where one letter ended and the next began. So they decided
to use the ‘o’ for every ‘u’ before an ‘m’ or ‘n’.
That's how we ended up with ‘monk’, ‘some’, ‘month’ and ‘mother’, although for today's purposes it is illogical because....
a) we now have printed books
b) most writing is done on a keyboard and
c) joined-up writing is not the most popular way to communicate.
Using ‘o’ instead of ‘u’ has just been handed down from generation to generation.