Merchants transact in fiat,
Most immediately convert BTC to fiat upon receipt making sure they don't lose money on btc instability.
Lightning network is even worse as the time locks insure no quick fiat conversion.
Meaning if a Merchant does use LN, they do it thru a 3rd party service that offers almost instant fiat conversion.
Actually, not most of them, according to
old news :
BitPay, the largest and oldest bitcoin payment processor with a daily volume of $1 million bitcoin transactions supporting more than 44,000 merchants, stated in an email exchange to CCN that more than 4,400 of their merchants keep all of their settlement in bitcoin, almost 18,000 keep some of their settlement in bitcoin while the remaining 22,000 convert it all to fiat.
If LN is to make inroads with merchants theirs needs to be price stability added.
Find a way to do that, and you could increase merchant adoption.
Without market control/manipulation or peg Bitcoin with more-stable asset, it's almost impossible,