I use apple pay... but I think I can help explain why this number is "low".
I had Apple Pay on day 1 (or pretty close too it). At that time there was one place I could use it. Subway. No local grocery store or any other location could take the contactless payment. In general I found this disappointing but expected no less with a new technology. The fight ensued where a bunch of other mobile wallets wanted to challenge and major retailers wanted their QR code system that was basically a joke out of the gate. That delayed the rollout of Apple Pay further in the US and was really a crap thing because the NFC cards and Apple Pay are basically the same thing so if the NFC cards were going to work... there was no reason why Apple Pay couldn't on these terminals. I grew sort of tired of the slow rollout and arbitrary acceptance at stores, so I basically stopped trying to use it in most cases.
by 2019 I used it for all my grocery purchases as the local grocery stores here finally caught up and they all now accept Apply Pay and did mark their terminals that they accept it. In 2015 the first time I paid with my watch people at the store would ask "what did you do?" They didn't know what an NFC payment was, how it worked, or what I was doing waving my watch at the terminal.
now in 2021, I have the first local gas station I have found that takes Apple Pay at the pump, our local grocery stores all take Apple Pay. Still can't use it at some stores around here but in general the acceptance has improved significantly. It isn't the easiest thing to use for the local drive through at MCDonalds. They ill accept it but it is a pain to have them hand the terminal out the window to wave your phone at it only to have it require your face with is not at the point of transaction which is out your car window..... (you get they picture here, it isn't "easier" than the card... it is more difficult and that is why it doesn't have a bigger uptake.)
so... things that would make it easier. totally ignoring cost.
put a fingerprint reader in the side button that. you need to double click for payments. Face ID is great for most things. but it sucks for window transactions, especially in the COVID era with masks. that needs to be streamlined so it can be as easy as the dunking donuts app with its QR code (yes I complained about the QR code before but the simplicity has its advantages.)
run a program to get those that accept Apple Pay to display their acceptance at the store / payment site.
Have (better) incentives. Plastic debit / credit card usage is a "habit". it is something many people do every day and Apple Pay is attempting to break a habit. in order to do so the benefit must be significant. under standard cases there is no benefit to Apple Pay. Same price for the same cup of coffee so to speak, where is the advantage? The card "just works"

, I don't have to ask any questions about acceptance and I pay the same price.
Ifi could carry everything in my apple wallet, and everyone would take it. my wallet would be a gone. but we are talking about drivers licenses, medical cards, car keys, house keys, and near 100% acceptance of Apple Pay. If we were there.... I would be damn happy but that isn't likely in my lifetime (I am 42). That is where they need to get to.