Because IBM picked the other chip for their flagship offering. It's as simple as that. If IBM had picked the 68K series a lot more funding would have been available to keep the Motorola chips competitive. In this branch of the multiverse they had to compete with the IBM/Intel combination, which with their massive sales of PCs and compatibles had both better margins and substantially larger volume. Motorola didn't stand a chance, even if they may have had bit of a head start technology wise.