Ann: Mukuyu-1 Sidetrack - Multiple Zones of Interest, page-418

The way I read the decision is they know there is additional positive formation below 3600m but the current location is not optimal to test it as it is pretty deep which will affect reservoir quality parameters. They probably already have an indication that this is the case. The priority right now is bringing HC to surface and the section down to 3600m has the highest probability of success for that. They don't want to jeopardize that mission by going deeper when they can test that deeper formation later on in a more optimal location where the reservoir qualities are likely to be better.