They wanted it mandatory because there it has no strings attached - it can be spent anywhere.
I doubt the IRA will be a "big win" for ordinary Americans. All the estimates I've read for its short-term effect on inflation (and if inflation is more than short-term, they're going to have much bigger problems than this bill solves) are near zero; even the White House doesn't dispute that.
The CBO estimates that about three-quarters of corporate income tax increases will be borne by shareholders, and one-quarter by workers. Many ordinary Americans are shareholders via their 401Ks and Roth IRAs, so increased corporate taxes are just more losses on top of the current inflationary erosion of the value of their savings. Corporations pay taxes that amount to less than the new 15% minimum because they get tax credits for useful things. Some of those will go away.
I'm skeptical that many ordinary Americans have a pot of cash handy to take advantage of rebates to buy improved appliances and cars and whatnot, unless driven by need (the old one is broken or nearly so). That can't simultaneously be true alongside the oft-stated claim that there, as well as here, most people are carrying substantial debt and living approximately paycheque to paycheque. Besides, giving people money to chase goods already in short supply is...inflationary.
Some people under the magic $400K line will see tax increases, Biden's promise notwithstanding.
File under: malinvestments, opportunity costs.