If I remember correctly, Radja said that he liked playing for Roma and living in Italy. He's been living in Italy (playing in Serie A) since ~2006..
Maybe he doesn't want to uproot his family and leave his friends behind for a move to England? It's not easy settling down in a new country, just look at Cuadrado. He didn't look like a confident player at Chelsea but as soon as he went back to Italy, he regained his confidence and played a key role in Juventus winning the title...
(Just to clarify, I would love to see Radja play for Chelsea, he's exactly what we need in midfield alongside Matic, but I don't see him leaving Italy for England, who knows, maybe Hazard can convince him during the Euros!)