Two reasons I can see:

1. In math they are worried about "gaps." In reading less so.

2. Each new math technique requires direct instruction.

#2 has been the issue for us. DS had 2 years of in-class "differentiation." In the first year, he got most of a year of instruction (dedicated teacher). In the second year, he got less than a third of a year of instruction (they didn't care as much).

DeeDee