I was visiting the Wallace collection in London (I can highly recomend it) and saw the sword of the Indian ruler Ranjit Singh (1780-1839). Its hilt was WALRUS ivory. Why would an Indian ruler use it - if Asian elephant ivory was not good enough quality I would have thought African elephant or Hippo ivory would be easier to get.

I don't think there were too many people hunting elephants in Africa during 1780 and 1893 and transporting it to Europe would have been difficult at the time.
The Asian elephant has much shorter tusks than the African elephant and some of the females don't have any. Therefore they may not have had a piece of ivory long enough for the hilt of the sword.
Walrus ivory was readily available at the time and were widely hunted. Could the Walrus ivory have been a gift?

Perhaps the rarer and more unusual the material, the more desirable for a ruler to own.

Found this on a Sikh heritage website

The sword even retains its original scabbard, carrying belt and straps, all similarly mounted in pure gold. The grip is carved from exotic and highly-prized walrus ivory, probably imported at vast cost from northern Europe, whilst the blade is of 'watered' steel, a cast-crystalline crucible type that is highly prized for its ‘watered' pattern, which still clearly discernable on the blade's surface.

Sounds very expensive.

I not think there would be much of a problem getting African Ivory - many modern poachers prefer poisoned arrows to guns and lots of slaves were going to Muslim lands (particularly via Zanzibar) - no point in them strolling out without a load that could be sold.

Forgot to say - the idea walrus was chosen because it was exotic seems convincing.