The scope of each site is determined by the community. Scope is a fluid, constantly-changing thing, and new meta discussions brought up by users can change bits and pieces of it over time, with the result decided by community consensus. Each community is independent from the others, and as such, they can shape their scope in whatever ways they want.
Physics and Mathematics both have very specific homework policies. Both sites require a user to show what they've done so far in an attempt to solve their problem, rather than just asking the community to do their homework for them. Many questions we get on those sites don't meet that particular hurdle, and are closed as off-topic.
One difference that I've noticed - and which is codified in the meta FAQ for Physics - is that Physics prefers conceptual homework questions, rather than questions about calculation errors. Doing computations is definitely a big part of physics, but there's a point where figuring out where a user went wrong becomes a simple math problem. Physics SE is much more focused on questions where the user has a specific conceptual difficulty about the physical aspects of the problem, and as such is quick to close many questions that simply ask where a user went wrong plugging in numbers.