Well maybe because he's a washed out player who lost all motivation? It is obviously not guaranteed a player does well in stronger league and can't do better in other lower leagues and vica versa but EPL, not only for the quality but intensity can cause players decline pretty quickly at some age. Rooney with his physically very demanding style peaked around 25/26 then started a steep decline, Sanchez in the best season for Arsenal at 27/28, also pretty reliant on speed and power. Hazard is a similar story imo, he's now 30 and EPL just sucked him out a lot. Also all those players were main players in their teams.
Going to Madrid is a specific situation too, you no longer are the only star man of the team so, you don't happen to get to the ball as much as before. Naturally you might lose confidence because of that and motivation if you're lazy fecker like Hazard. For him going to Madrid he must have thought he's done it and only shows how unfit he showed up. Also their ageing squad who won it all meant noonereally worked harde nough for the star players like it was at CHelsea for him.
No wonder they are where they are with a manager who solely motivational one and has no real system to clutch at when things go bad. Griezmann as a transfer withing the league is another example, a player who peaked and really became one of many in an aging/no longer top squad build around walking Messi.
I think we will see that more players peak early and fail at bigger stage because of what I mentioned. Football is phisycally and mentally more demanding than ever. Only the best will last longer on top.