Best guess - Network effects (of more than one kind but rooted in very low marginal costs to produce one more unit) tend to favor monopolies - but monopolies tend to keep prices very high until they have at least one competitor. Then they lower their prices. But the real reason may well be that the government lands much harder on monopolies with no competition - so it's actually more profitable in the long run to have one competitor with 20% of the market than to try to grab the whole market. High tech monopolies in particular have been know to boost or subtly subsidize these pet competitors to keep them around, and the AG's attack dogs away. I suspect we'll see more thorough regulation in the next few decades so this somewhat cheesy strategy might stop working.

The real duopoly is x86 vs ARM - Intel and AMD were a forced dual source, long ago. So why doesn't MIPS get more love? Does RISC-V have a chance? Intel's disinterest in low-powered chips opened up enough space for one competitor in a slightly different niche.