I think ML is a bit niche in terms of overall number of programmers doing it? Meanwhile, making websites has turned out to be very popular.

I suspect the response rate there is actually skewed high by the large chunk of students.

Regarding the CPU use and hosting cost, it's probably not an issue for most websites. Usually you end up database bottlenecked much sooner, and that doesn't kick in until you're on >100s of millions of rows.