Off the top of my head without going to look at a load of papers:
All languages simplify over time in many ways - e.g. originally we had one word that meant heart, mind, spirit and courage, but over time we added more words and those other meanings split off leaving the original word to semantically drift until it ended up as 'mood'.
Simplification takes many forms, but one of them is pronunciation, leading us to -
NRP - which stands for Non Regional Pronunciation - has historically been a sort of classless variety of English that eschews dialectal markers, including those of RP. This sort of spread around in the 20th century and was used by broadcasters to replace RP (received pronunciation).
Then a variety called Estuary English (coming from Southeast England funnily enough from the London area) started to become more prevalent and take over. This variety borrows some of its phonemes from Cockney English which is why we get more glottal stops, and yod dropping and some other things I can't think of right now.
Most linguists would agree Estuary English is now NRP, that one has overtaken the other.
Edit - the reason for the spread is technology in that everyone can now watch TV and videos and shorts online, and also we have far greater population movement now so it's much easier for changes to spread than they used to be.
So that's why these pronunciations have spread, and returning to the idea of simplification, it's simply easier to pronounce tune and lute without the /j/ phoneme. People are like water, we will always take the path of least resistance.