a) These companies are much bigger than they were, which increases the impact a programmer can have. This means the amount of money a programmer can bring in for one of these companies is much higher than previously, and so they're willing to pay more.

b) Previously, programmers were capturing a much smaller share of the value they were creating for their employers. Either through explicit collusion to keep employee salaries down (https://en.wikipedia.org/wiki/High-Tech_Employee_Antitrust_L...) or because the threat of startup founding (which is both easier and more normal relative to 10y ago) has pushed salaries up.