Likely this. UA had a limited code share with 9W until 2014 (when India was downgraded to FAA safety category 2) but didn’t renew them when category 1 was restored. But they continued the interline (preference over AI), and redeemable miles earning until 9W’s tie up with DL.

I wonder if UA (and possibly AC) has exclusivity regarding North American carriers, or will UK become everyone’s code share partner and also tie up with DL?

Regardless, I much preferred flying BLR(9W)-DEL(UA)-US over LH’s horrible skimlines on their 747s, and hopefully UK will fill 9W’s void nicely.