That's completely unrelated to the popularity of card payments though. It's a denomination that's never given out as change or by ATMs

See also: https://www.bbc.com/news/business-48993008

which as well as explaining the background to the note and debates over whether it should be taken out of circulation also points out the context the average British adult was making about 20 cash transactions a month as of 2018 (none of them involving £50 notes)