Kante was getting a lot of stick during Lampard's tenure but this was mostly because Lampard played suicidal football - he set us up in a 4-3-3 then bombed both fullbacks forward, so when defending we were something like a 2-1-2-5. Basically he asked Kante to mop up literally everything - and this was impossible, even for Kante. This is why we got counter-attacked to death - god only knows how many draws we turned into losses because we played short-sightedly and impatiently.
Tuchel basically realised that having Kante sweep horizontally is fundamentally a misuse - Kante's best attribute is his ability to press high then recover. Thus using him as a rover in a double pivot makes far more sense.