Because the word "couple" may as well have 5 tongue twisting consonants in a row, far as a native Japanese speaker is concerned. there is no such thing as the "cuh" sound nor the word "up", let alone the "pl" sound.
But this is obviously uploaded for a non-native audience. Similar to how the world collectively decided Adolf is no longer a good baby name, the English world has decided CP doesn't stand for anything good, ever.
Probably the best compromise would have been to call it a ship or a pairing with a TL Note for people who dont know what either of those are (which wouldn't be a loss since you had a note in there anyway).