It's been a trend. Not a good one IMO. I don't think going so fast, more often, is good for the horse at all. But, commercial breeders push for this because it's what supports their business model.
We need to take away 75% of the money for two year old races and allocate it to races for horses which are four years old and older and do the same with 50% of the money for three year olds). It would change the economics of retiring horses. This would lower the value of yearlings because there would be more risk and it would change the financial consideration of retiring a horse vs. continuing to race.