Because RC5 is trying all the keys one at a time, not trying to crack it. 64bit encription would fall in days under just Phlumps machines, with actual cracking tools.

RC6, a cryptographic algorithm, is specified as RC6-w/r/b, where w is word size, r is the number of rounds and b is the length of encryption key in bytes. RC6 is the successor to RC5. It is one of the ...