Well according to Wikipedia https://ja.wikipedia.org/wiki/%E5%BD%93%E3%81%A6%E5%AD%97 沢山 is just 当て字 (ateji), so the answer to that would be: Because the 音読み of both of the kanji fit. Not because of any of the meaning of the kanji itself.
However some people seem to be under the impression that it is not 当て字, see here:
https://detail.chiebukuro.yahoo.co.jp/qa/question_detail/q12123570802 (and the related questions also)
Someone says it's 当て字, someone else says it isn't, someone says it's a 漢語, someone else says it isn't etc. Of course they're not really experts on kanji there, just native speakers.
Personally I would just think of it as an 当て字.
Why Japanese use a mountain name to describe "a lot".
Anyone know its origin?