I think Modric deserves the award to an extent. He may not be have been the highest goalscorer for the year but his contribution to football this year has been enormous.
Firstly, Luka Modric won the UEFA champions league with Real Madrid. His role in the team cannot be overemphasised. Those killer passes delivered with inch-perfect accuracy, his vision and ability to detect the run of the forwards. Modric stands above the others in that regard. All of that effort culminated in the Los Blancos outfit winning the champions league for the third time in a row.
Also, armed with the captain band, Luka Modric was the driving force of a Croatian team that did not have the biggest of stars. Players like Lionel Messi and Cristiano Ronaldo can change any game they are in just by their presence on the pitch. Yet, Croatia never had the fear factor. They relied on, perhaps, their only two or three big whips in the persons of Ivan Rakitic, Mario Mandzukic and, of course, Luka Modric. It was the later that actually spearheaded their campaign. Game by game, they weathered the storm until they finally reached the final.
Among all football players, it's safe to say that Luka Modric contributed the most and achieved the most.
With all due respect to Antoine Griezmann, Modric was the best player for 2018.