Virat is way more experienced in leading India against competitive opponents like England and South Africa, away from home. India dominated South Africa in the limited overs format and lost 2-1 in Tests but what we are focusing on is the ODIs.
In England, they did win the T20 series but narrowly lost out on the ODI trophy, losing 2-1. What's even more important is that Virat has the experience of leading in tough and swinging conditions and then coming out on top. This would really matter in the 2019 World Cup. Rohit Sharma, on the other hand, has only captained India on Asian tracks which are of little significance, keeping the 2019 World Cup in mind.