The reasoning is generally that using SizeOf implies one is doing unsafe things. Why do you need the size of something in managed code? What would you do with that information?
Not that I necessarily agree with the decision myself, but that's the argument made by others.
I cannot get behind that argument, it seems to me that it ends up needlessly marking work as dangerous and in need of proper review when it's actually not at all remarkable and so you teach people these reviews aren't really important, it's probably just another false positive.
Rust spent lots of effort on making sure these lines are in the right place. The current work to try to "make C++ safer" is likely to ban taking a 32-bit unsigned integer and treating that as a 32-bit IEEE floating point value because it can invoke Undefined Behaviour in their language. Meanwhile in Rust that exact feature is not only safe, it's a pure function named `f32::from_bits` because 0x40000000 literally is 2.0, it's never any other value, there's no actual danger here, you just need to fix your programming language but C++ would rather waste your time on this non-existent problem.
Not that I necessarily agree with the decision myself, but that's the argument made by others.