PCs had a different (open) hardware architecture using chips, chipsets, motherboards and other components from a variety of manufacturers (Intel, AMD, Cyrix, Nvidia (later), etc).
Macs had their own architecture from top to bottom from the Motorola chips (680x0, PowerPC, etc) to even the system interconnects.
This custom hardware slowed down the innovation at Apple and drove up the cost at a time when horsepower was really important.
What killed the Mac was cost and scale so when the final nail in the coffin came with Windows 95 (which exceeded the technical capabilities of MacOS in about every way, UI sensibilities notwithstanding) the disparity was vast.
Now? The upper bound on power isn't cost or availability, it's something far more immovable: battery life, size and weight. All the while, CPU differences are largely nonexistent (Snapdragon, Apple's A4/A5, etc).
So, to be clear, you could buy a better (specced) PC for less money than a Mac. Now you can buy a worse Android phone for less money. There's obviously a market for low cost handsets but I don't think anyone can argue that any Android hardware is significantly better than Apple hardware in any way that consumers actually care about.
You see this in, say, the TV market too. If the cheaper but worse hardware argument held then Chinese TVs would kill off higher quality brands. I once heard there are three tiers of TVs: Japanese (top), Korean (middle) and Chinese (bottom).
Chinese TVs are obviously a big market but they aren't the entire market. Nor are Samsung (Korean), Sony or Panasonic going anywhere anytime soon.