I think it's called chasing the mighty dollar. There is literally a point somewhere around 2006-8 onwards in not only R&B but also in Hip Hop, where the soulful feel and sound dropped out of the music almost completely and apparently became 'unpopular'. It's the same time that people say the Neptunes fell off, Just Blaze wasn't as popular, Dre's productions started dying up, Timberland had been quiet for a bit. It's right before the time when Timberland dropped Sexy Back with Justin and triggered off the whole EDM phenom in R&B. For whatever reason you all of a sudden couldn't cross over to mainstream with a typical soulful and funky R&B or Hip Hop sound... Personally I smell a media industry conspiracy, as at just around that point everyone was starting to say Hip Hop and R&B was the new pop.... and just think of that whole Neptunes, Timberland, Dre, Just Blaze and Swiss Beats error... You couldn't go five minutes without hearing one of their beats on mainstream. I think the industry got scared of the revolution, and all the black millionaires that the period was creating.
I remember hearing Tim on a interview over in the UK with Hip Hop Radio One DJ Tim Westwood, and he was going on about how the music industry wasn't giving him his props, and that he was about to come back and stun the whole game with something new.... it took a few months and then Sexy Back came out and I was like what? What's this? Where's this going?
Next thing it was Black Eye Peas, Neo, Rhianna, Pitbull, Akon, Usher, Chris Brown....... All of a sudden R&B stars were chasing EDM producers to get beats for their singles. At the same time Hip Hop went over to the South Trap sound and never came back.
The thing is although I think the media (mainly the rock and pop industry) tried to send black music back to the ghetto and out of the pop charts, the fact that all these artists switched up and adopted a euro centric genre means that the millionaires are still coming thick and fast and the influence is back on mainstream radio. Also think about it, the sound palette of Trap is basically the same as EDM, loud bright and synth driven, except more bass but still without soul.