It's not necessarily a demand issue. It may be that companies are abusing the system to gain more such workers. I've certainly been part of hiring searches where qualified US candidates didn't get the job. It often came down to money. I'm in a higher cost of living area, so salaries tend to be higher than average. But the company was offering lower than the national average, much lower than the local average. The candidates from the US were often offered the job, then turned it down because the pay was so low.

"I can't afford or don't want to pay market rate salaries" isn't supposed to be a valid reason to grant an H1B visa.