Very easy: convenience, security, and cost. You are right that consumers will care little about the cost aspect, because it is largely hidden from them (even though they pay for it indirectly) but they do care about convenience and security. Online payments are actually not really that convenient. That's why Amazon is so popular because they sort of shield you from having to setup payments with each of the merchants they proxy for you. This convenience comes at a price; Amazon is not a charity and highly profitable.
Copying credit card numbers all over the place is tedious and risky and people are kind of nervous about it for valid reasons. Swiping cards like a caveman is already a thing of the past and also not that safe. There's a huge amount of friction around payments for both users and merchants both online and offline. Even cash has a cost for merchants.
Contact-less payments with cards or phones are rapidly replacing cash payments (more convenient). But of course online you basically still get to copy credit card details around or deal with middle men like paypal or one of the many direct debit systems in many countries.
BTC is nice as a gold alternative and similarly tedious to transact with. It's useless for payments as transactions are extremely slow and rate limited to a ridiculous ~5 transactions per second globally. Price volatility means that it is hard to price things in it because you have to update the prices continuously. Ethereum is useless for the same reasons (in its current form).
Libra will require a bit of selling and educating obviously but actually comes with all the tools needed to make that easy. E.g. micro payments and tipping are very easy to implement on it and you could use that to incentivize users with e.g. bonuses, tiny refunds/rebates, etc. Integrating payments into chat removes a lot of friction for the user. And once there are enough merchants adopting this that incentivize users to use this, adoption could be quite rapid. Having low transaction fees will make this very interesting for merchants.