In the past, I gather there was reluctance to publish methods because there was competition for support from wealthy patrons. So mathematicians felt they had to produce exciting results without giving away too much about how they got there.
Is this obscurity restricted to mathematics? There seems to be a general resistance to plain English in academic writing - from some reviewers, publishers and even readers.
"It was determined from the research protocols outlined outlined in this paper that ..." = "We found ..."
Some people feel that it doesn't sound"serious" enough unless it is hard to read.
(Disclaimer: I am not an academic, but I have heard this from lots of researchers and editors)