Upvote:2
Indian nationals need a visa to transit airside in the United Kingdom.
While there is a long list of exemptions that you may wish to review, nothing currently given in your question shows that you qualify for any of them.
So, the airline was correct to deny boarding.
As Burhan Khalid noted in a comment, you were most likely booked on Kuwait Airlines flight 102, which flies JFK-LHR-KWI, thus triggering the transit visa requirement. You probably want to be on flight 118, which flies nonstop JFK-KWI. This flight is only scheduled three times a week, though, which is probably how you ended up on 102.
If you're still at JFK, the next flight 118 leaves in a bit over two hours from now...