A big reason they said they were raising the deal previously was the cost of living in the bay area and increased cost of starting a company today[0]. Are they anticipating that those costs are going to come down now? They suggest that this decreased amount will allow them to fund more companies, and hint that it has to do with economic conditions as well. But, if that amount of money is still needed to live in the bay area and work for 6 months, then wouldn't it make more sense to decrease the number of companies funded as to not potentially handicap startups with not enough money?
[0]https://blog.ycombinator.com/new-standard-deal/