I'd have to research it to answer more definitively, but its probably because of the way we have feature code in separate projects and use post build events top copy the files up to the web project at build time. The debugger can break and it can find the line of code and step through it but since the code doesn't really live in the Web project where we are debugging from it can't edit the code and continue. I really don't use edit and continue much but I suspect it should work for code that lives directly in the web project.
Best,
Joe