One reason is probably after Jan 15 you will be filing your return with a payment or after Jan 15 you will be paying a payment with an extension.
See page 2 when to file, https://www.irs.gov/pub/irs-pdf/f1040es.pdf
* You don’t have to make the payment due January 15,
2020, if you file your 2019 tax return by January 31, 2020,
and pay the entire balance due with your return.
