Because the stimulus checks are coming soon, and it you file your 2019 taxes now you may still have a chance to make certain elections that will affect the check amount. Lacking information from the IRS, as our government has become prone to do as of late, people make their own best guesses, which may be irrational or even paranoid.
For example, another branch of this question relates to the claiming of dependents, is it tax year 2019 or tax year 2020 for which a person claimed as a dependent on another person's return is rendered ineligible for the credit? Reading the language of the law, it would appear to be tax year 2020, although tax years 2018 or 2019 will be used for determining if an advance payment will be made. How permanent the advance payment will be, and whether the credit will be a refundable or nonrefundable credit if claimed later on the 2020 tax return, is a fundamental consideration in the tax elections to be made now.