Edit/note: At first I thought you meant a USDC/USDS pair, but after thinking about it, I think you mean both BALN/USDC and BALN/iUSDS pairs. The below still mostly holds true.
So, this is my understanding, anyone with developer/coding knowledge can add a pair. I don’t know what the framework/requirement is to do that, but I believe it is possible.
The 3 new OMM pairs were added by the OMM team (I believe), and incentivized by them (you stake your LP tokens on OMM to get OMM rewards for supplying liquidity).
The bnUSD/iUSDS pair was added by balanced (I believe), and BIP5, a proposal currently in the voting phase, includes adding BALN rewards to liquidity providers of that pool.
I believe if a pool is not incentivized, then the Liquidity providers would only get a portion of the fees from the pair for providing liquidity.
The issue for Balanced adding the pair you mentioned, USDC/iUSDS pair, may have to do with the framework/requirement for adding the pair.
One possible issues could be that a minimum amount of liquidity for the pair is needed to add a pair (I don’t know though). Balanced does not have much USDC or iUSDS on hand, only what has been delegated to the DAO fund and Emergency Fund (I believe). And it may require a vote to use those funds in this manner.
Edit, an issue for BALN/USDC and BALN/iUSDS pairs specifically, would be that it devalues our product. I’m not saying that I don’t think those pools could be added, but that I wouldn’t vote to incentivize them (as I would want people to value bnUSD more than other stables).
These are just my guesses. I’m looking forward to learning more.