Biggest reason I can think of is that airlines have so heavily invested in business class over last 10 years or so to the point where business is nearly as good as first class has been at a fraction of the price.

I've had the privilege of flying international first and business class flights several times over past few years using points (including Singapore Suites), and business class has gotten so good (lie flat reverse herringbone configuration) that there's hardly a difference between the two except a slightly bigger seat and marginally better food but 3x the price.

We are living in the golden age of business class travel.