By convention the Test captain is always considered the senior captain and the ODI the junior one. When the senior captain falters and the junior captain does well, then there’s pressure on the senior captain to step down. Here the senior captain is doing great and the junior captain is faltering. Once you’ve handed Kohli the Test captaincy and he’s winning you Tests and hitting ODI centuries and blasting T20 bowlers at the same time, it’s quite difficult to not hand him complete captaincy. The BCCI always botch up handovers. In fact they did not make Kohli Test captain out of planning but were forced to do so when Dhoni quit midway through a series.