We can suppose some ideas together, but only you or your tax specialist can answer some or most of your question; you did not provide sufficient information to get a really specific answer.
============
Basically, the IRS established its estimated-payments system for folks that the IRS believed to be at risk of being unable to pay at tax payment time. Independent contractors are notorious offenders, whereas estimated tax payments for regular hourly or salaried W-2 employees, are withheld by their employer, per IRS rules, at every payday.
So it could be that the IRS noticed that you had a "significant" amount of income (I do not know what threshold the IRS might use to flag an amount, or a ratio, to be "significant") that you reported for 2019 that was not reported for 2019 on a W-2 from an employer. Or perhaps the IRS simply noticed that your 2019 payment due at filing was unusually high when compared to folks with steady income who make estimated payments, whether self-employed or W-2 employed.
Clearly, something about your 2019 return flagged the IRS, and now the IRS is hinting, perhaps demanding, that they'd like to see you make estimated payments.
I do not know the estimated payment rules (you should look that up yourself); however, I am pretty sure that if you're "supposed to" make estimated payments, but then at tax filing time you do not owe (or get a refund), then you're not going to be hassled by the IRS for not making estimated payments if you were "supposed to".
When I was self-employed, I paid myself through the same payroll service used for my employees, so that I did not have to cough up my own quarterly withholding through estimated payments. That smoothed out the cash flow and avoided the extra effort to do the estimated payments.
