As far as I've understood, it's complicated but I think some or all of the following reasons are involved in Intel starting to lose its foothold on the market:

As far as I've understood, it's complicated but I think some or all of the following reasons are involved in Intel starting to lose its foothold on the market: - AMD got a new very competent CEO (Lisa Su) that managed to save the company and turn its fortunes around; - In the last decade, Intel seemingly forgot what its core businesses were, and instead invested lots of its time and resources to try to break into new markets such as smartphone CPUs and modems (failing miserably at that); - AMD is fabless; they got rid of their expensive fabs in the '00s by spinning them off as GlobalFoundries. When TSMC perfected its 7nm node, they just jumped manufacturers; - Intel has lost the fabs lead it had years ago; I still remember them releasing 22nm chips while AMD was still on GloFo 45nm. Intel has been stuck on its 14nm node for years, and they haven't been able to release 10nm chips until last year; - AMD saw a clear market interest for CPUs with a higher core count, and they managed to deliver a "good enough" CPU that appealed to buyers in a stale market. The Ryzen R7 1700 had slightly lower single-threaded performances than Intel, but it offered immensely better multicore performances than its Intel equivalent for a comically low price point, on cheap motherboards and with its stock cooler; - Due to almost complete lack of competition from AMD for almost a decade, Intel got greedy and CPU prices skyrocketed; in 2017 an 8 core HEDT CPU from Intel was sold at a price close to $1100, which is almost as much as a whole R7 1700 setup costed. AMD had nothing to lose and no risk of market cannibalisation, so they could just price their CPUs at much lower price point than Intel; - Ryzen is arguably a good design, and AMD's idea of making high core count CPU by interconnecting small quad-core CCXs instead of making huge dies like Intel Xeons was a bet that really paid off. Operating systems already more or less supported NUMA-like architectures, and AMD got from this a very scalable architecture. Like Threadripper has shown, they can just "glue" CPUs together; the final result is a much bigger package and a huge socket, but nobody really cares about that.