Let’s examine which liquidity pools have the highest liquidity and volume. The pool with the highest liquidity is the EGLD - MEX pool. The reason for that is simple. The correlation between egld and mex is very close to 1 (highly correlated), because both are coming from the MultiversX team. High correlation between two assets means minimum impermanent loss of that pool ( both go up and down at the same time, at same percentages, or very close most of the times). But even thought this pool has the highest liquidity, the highest volume is coming from the EGLD - USDC pool (more than 50% of the volume). Part of this success is the popularity of egld as a coin. But in my opinion it’s not only that, because more pools are paired with egld, and they dont have this success. When traders trade, prefer to have one token “stable”, so that when they make any profit from the more volatile asset (for example egld), they can lock profits by trading back to the stable coin. That makes sense right? Now I am wondering, why there are no more token pools paired with USDC? At least the ones that are getting some attention and are getting more popular approaching EGLD popularity( for example HTM). I think if you create more pools paired with USDC you will see the same phenomenon that you see in the EGLD - USDC pool in other pools as well. Correct me if I am wrong, but I think this argument makes sense. Traders provide 99% of the volume on the exchange. Therefore, give the traders what they want and need. Liquidity providers will see after the high fee rewards on these new pools and they will provide liquidity to them. 
they are not at all closely correlated. Just because both tokens were issued by the same issuer, doesn’t mean the tokens are closely correlated. MEX is down -99,999% or so against EGLD. That is anything but closely correlated. USDC to USDT is where you would use the term “closely correlated”.
this pool has among the highest IL on xExchange
Incorrect. It is because EGLD is a much more traded asset than MEX or any of the ESDTs on xExchange with multiple price fluctuations per day (high volatility) which results in arbitrage and henceforth high volume. Additionally, this pool has a LOT of TVL as well.
The issuers of tokens have the option of pairing the token with USDC on xExchange if they like, but many do not prefer to do so. Others did.
Part of the reason for this might be that they hope to get additional upside of EGLD of course.
Other part of the reason: it is very usual and common to pair your own token with the coin of the network you are on (for example, on Solana usually everything gets paired with SOL, on Ethereum with ETH, on Sui with SUI and so forth).
Finally: this one is kinda special to MultiversX: USDC supply on MultiversX is very low (i think currently at ATL last time i checked defillama). I don’t know why we are losing so much of USDC (people bridging it out to other chains) but that’s besides the point. The point is: it is happening. And that means less and less USDC for liquidity on the chain compared to possible liquidity in EGLD (less liquidity possible for your token to have as TVL). But also it means it is hard to get USDC on MultiversX. To get USDC you either have to bridge it yourself or sell EGLD to buy USDC and then use that USDC to buy the token you so want.
So it just becomes much more inefficient to buy tokens when they are paired with USDC (additional swaps involved, more fees, less liquidity…). Also EGLD is much more fungible and can be transferred to CEXes and sold there, or even used to charge the xPortal card with, but USDC cannot. As it’s just a wrapped Bridge token with low liquidity.