Because there really isn't that much to Kante, to his credit he's defensively impressive and possesses one of the best engines around but he's not particularly good with the ball that any team looking to play possession based football will play him as a holder and he's not anything special going forward.
In a 3 man midfield he'd be a very good player, like Napoli's Allan or PSG's Matuidi, a shuttling midfielder that offers a bit of everything. Madrid need a player like this to support Kroos & Modric, but we already have Elneny & Ramsey who are of a similar mould.