Can’t remember the full details of the deal, but I seem to recall a story about how Apple approached Intel to manufacture a low-powered processor for mobile (for the first iPhone) at the time. Intel didn’t see money in mobile processors and passed on the deal. Additionally, for years, Apple asked for more powerful chips for the MacBooks. At the time, the iPads were surpassing MacBooks in speed on some tasks. Finally, Apple decided since they were already designing their own silicon for iPhones and iPads that they might as well just do the same for the MacBooks as well since Intel couldn’t keep up.

Again, this is largely from memory. I can’t remember the source, so take it with a grain of salt.