Because there were others out there
Ah, so Pfizer was afraid their vaccine wouldn't be ready before Covid disappeared but Trump reassured them?
As @AsherO mentioned, Wall Street apparently felt it was a big deal.
Let's look at all the things 'Wall Street' thought was a big deal for Tesla and GameStop.
Federal government taking Pfizer seriously is a good soundbite, not something that has a material affect on bringing a vaccine to market.
It cost huge money to develop a vaccine, especially on a platform that hasn't been brought to market yet. Having ~$2b in upfront orders is a significant carrot.
I'm disputing that $2b in upfront orders is carrot. If you mine gold, having upfront orders for that gold is irrelevant. You either find it, or you don't.
Please explain how having upfront orders for something they can obviously sell helps Pfizer?
That's before you dig into the timeline and question how much of the vaccine and trial were already underway and budgeted by the time the $1.9B contract was signed.