I'm not certain, but a quick google shows that the dividend rate of the S&P 500 varies quite a bit month to month. It's my belief that VTSAX chooses to pay out all of it's accumulated dividends in a pass through manner. I don't know when VTSAX quarter ends are, and I don't know if there's any sort of lag between quarter end dates and dividend payment dates.
Based on all that, I'd guess that VTSAX is paying out a lower dividend because the stocks it owns are paying out lower dividends. Why companies are lowering their dividends I have no idea. Maybe as a buffer against economic uncertainty, maybe a reaction to the new corporate tax rates which means that investing in the company's growth makes more financial sense, or maybe something else entirely.
(Yes, I know the S&P 500 isn't equivalent to VTSAX, but it covers about 80% of the capitalization, so it's a good rough approximation and it's easier to get data on S&P 500 metrics.)