In this, I'm not referring to 'retro remakes', like the C64 DTV which was a licensed machine in the late 2000s, but the knockoff/unlicensed clones that were endemic with 'price leader' computer companies in the 1980s and early 1990s. What kept the Commodore 64 from being undercut like all the rest?
The original Apple II was built entirely out of standard, off-the-shelf components: a 6502 CPU, RAM, ROM, and 7400-series logic chips. I'm not familiar with the TRS-80 or Spectrum, but I know that the IBM PC and XT, which were also widely cloned, were also built out of off-the-shelf parts.
The Commodore 64, on the other hand, had custom integrated circuits for video, audio and I/O that a cloner would have had to reverse engineer in order to build a compatible machine. It wouldn't have been an impossible task (Nintendo's Famicom/NES, which was of comparable complexity to the C64, was successfully cloned during its lifetime) but the barrier to entry was substantially higher than with the Apple and IBM machines.