Yield farming also referred to as liquidity mining, is a way to generate rewards by locking up LP Tokens. The vast majority of yield protocols focus on one DEX, and offer several pairs.
At YieldFarting.finance, they decide to focus on one pair, one dex. It is not necessary to propose pairs that are quite useless. It is necessary to concentrate liquidity on the most important pair (FART/BNB) and to expand as much as possible within the BSC ecosystem.