debug is actually in the 32-bit version of windows 7, I believe. It's just not in the 64-bit edition; same story for Vista and XP x64.
Almost surely because of two things. First, if they did include it, it would be useless since you can't run it. That's the only reason needed, really, but the second was that it was pretty much a cheap 16-bit assembler/disassembler, which dealt with 16-bit code. And since 16-bit code cannot run on Windows x64 that's a second nail in the coffin.
Notice that there are a lot of other "missing" files. x64 versions of windows don't have program manager (progman.exe), File Manager (winfile.exe), windows write (write.exe) (although, write.exe is aliased to wordpad.exe in all versions of windows since windows 95 through registry magic, iirc). Those utilities we do still have are 64-bit; find, sort, etc come in two versions on x64 machines; a 64-bit version in the C:\Windows\System32 folder, and a 32-bit version in SysWow64. Some programs were phased on simply because there was no point keeping them around. Very few people used Program manager, or File Manager. In fact people that even knew they existed beyond windows 3.1 (if they knew about them at all) are few and far between, so it was not worth the effort to get them compiled even as 32-bit programs.
I think the closest one can get to Debug's functionality on a x64 system would be to use GNU Debug (gdb).
