1) Early on, Intel was too focused on raw speed, at the expense of power management. This meant that they were never a viable contender to provide CPUs for the iPhone, or subsequent android smartphones as well. Because of this, Intel was effectively shut out of the growing smartphone market, at a time when PC sales were starting to stagnate. They basically locked themselves out of the next big thing.
2) Because of their early financial success in providing cheap x86 processors for servers and data centres, Intel never felt the need to innovate and move beyond x86 instruction. It's classic disruption theory - a company doubles down on that which made it successful in the first place, at the expense of missing the next big thing.
Intel now faces threats on multiple fronts.
1) They have already lost Apple's business.
2) AMD is proving superior to Intel, performance-wise.
3) Cloud providers like Amazon are starting to design and manufacture their own ARM chips, threatening their dominance in this aforementioned area.
4) People are holding on to PCs for longer, which means stagnating or even declining computer sales, which means fewer processors sold. Which is a double whammy because processor design is an extremely capital-intensive process, which is typically offset by huge volume sales. This means that Intel has even less incentive to work on improving their processor designs, if they didn't think the market was there for one.
5) TSMC is eventually opening a fab in the US. And guess what (1) and (3) have in common? They all rely on TSMC to manufacture their ARM chip designs.
The TL;DR is that what started out as Intel's greatest strength, that they integrate both design and manufacturing, is now becoming Intel's biggest weakness, because nobody cares about Intel's chip designs, since companies are now increasingly moving towards designing their own processors that are optimised for very specific tasks (which Intel will never be able to do for any one specific company). And to add insult to injury, they are contracting TSMC to mass-produce these chips for them.
This is basically what Intel has sown.