Why would banks not want to lend, under Federal Reserve's Main Street Lending Program?
I saw these two answers on Reddit or Quora, but I can't find them now. Just edit this if you do.

Because the potential losses are much bigger than 5% if you read the program. Consider the pari passu clause. In a liquidation, what's the collateral that the Main Street loan has a claim to? Collateral that is securing any existing loans you’ve granted! Thus your loss can easily outstrip the 5%, as it eats into existing collateral and increases a bank's deficiency.
Even if the bank just loses 5%, the people needing this money are going to be very risky and banks can focus their time and efforts on way less riskier programs. Contrary to popular belief, a bank doesn’t just magically get bailed out with zero pain anytime they lose some money. It’s still a 5% loss which on loans of this size are significant once they get added up. Moneys have much less risky and better ways to sell their money right now.

That is why the loan program should have been more like FAFSA that use IRS data to verify businesses and grant them an amount. FAFSA get to pick a bank to service or another servicer. This is how the student loan program works. If the loans were based on credit alone then no one would get them. Doing this through the banks was a horrible idea. If you grant businesses an amount, then they are the customer that chooses the bank that gets the servicer fees as the loans are guaranteed. Not the other way around where banks are the gatekeeper to this money.
It is amazing how bad banks screwed up in the last bailout which they used just for themselves and to shore up capitalization/leverage. We were stupid to trust the banks this time ago, and by design are helping larger companies by the cronies.
The market is efficient. You have to set the right incentives and targets though.
Banks are in self-preservation mode. If they were cut out and had to go court customers with free money in guaranteed loans with windfall service fees from the bailout they would, though since we let banks be the gatekeepers to survival again they get to keep free money and they will cut risk.