I think the biggest reason of cart abandonment (of next to the price/cost) is complicated forms and checkout process.
Most of E-commerce sites have really bad UX in checkouts. None want to fill all this info, Name, Email, Address, Country, payment infos. Especially from mobile phones. Me personally really dislike to type on mobile.
However, Apple Pay, Gpay, Stripe and similar companies trying to make that processes much simpler.