Probably because poloniex ranked too low to be on the first page? They were at the second page on coingecko.
Poloniex is ranked at 109th on Coingecko.com and on Coinmarketcap, its rank is 81th and on pages #2 of both marketcap sites.
https://www.coingecko.com/en/exchanges?page=2https://coinmarketcap.com/rankings/exchanges/Poloniex was once at the top but have lost most of their userbase because they fucked up and better competitors entered the market. Coingecko also does place a lot of emphasis on their trust score metric in terms of ranking and poloniex is at 5/10. It also appears coingecko revamped their rankings because I'm pretty sure binance was at the top, last time I checked lol.
They lost their user base to Bittrex exchange that was closed last year and Binance.
The reputation, trust score from Coinmarketcap or Coingecko is based on their own formulas and can be not neutral ranks like different ranks for Binance (global) on Coinmarketcap and Coingecko.