Petroleum products are those products which are subject to maximum tax both by the Centre and the States. Centre imposes excise duty, States impose value added tax. 

The revenues are so much that both the States and the Union are not able to agree to bring them under GST. It is a perception that there may be loss of revenue to the Centre and the States both if petroleum products are brought under GST. 

For this reason, it has been specifically provided under the Constitution, that petroleum products are a part of GST but that situation will not become effective unless there is a clear notification by the GST Council which specifies that yes from so and so date petroleum products will be subject to GST. 

That date is being withheld so long as the Centre and the States are able to converge on a particular revenue collection target which is met otherwise and once this happens, GST Council will notify the date and petroleum products will be brought under GST.