Just a personal theory: Maybe because cultural issues were much more discussed since around 2014/15. I feel like the Democratic shift towards the left certainly as alienated some voting bloc in the entire country, with Cubans among them. I'm not 100% sure it's just that "socialism and Castro regime" stuff. Nobody who pays a little attention believes Joe Biden is a socialist. Also, I doubt Mike Bloomberg would have done better among them. However, Cubans are socially conservative and as the economy has vastly improved since Obama's reelection, these voters shifted more vote on cultural issues these days, where they agree more with Republicans.
One thing that is worth noting is that pre-Obama in 2000 and 2004, Cubans often voted simillar to how they do now. I def think Obama and the reccession played a role in shifting Cubans left for the decade.
