This is a weird question but I can't resist. I had to ask. Why don't we receive reputation points when some posts get upvotes in Mathematics Meta Stack Exchange?
Reputation is a rough measurement of how much the community trusts you; it is earned by convincing your peers that you know what you’re talking about.
Knowing much about mathematics does not equate with knowing much about Mathematics Stack Exchange. Of course, these are not mutually exclusive, but they're two different things and we don't add your Physics Stack Exchange reputation to your Mathematics account either (apart from the association bonus).
Also, there are some users who are very knowledgeable about mathematics, but have 'unpopular' opinions which tend to get downvoted on Meta. It doesn't make sense to deduct those from their main reputation, does it?
An alternative would be to have everyone start at 1 meta reputation (just like everyone here starts at 0 badges) and apply up- and downvotes as they would on the main site. This is how Meta Stack Exchange works, but that has its own problems and some people have asked to abandon that system.