Oven was cooking the entire sea, Big Mom can create storms and control the sea and the territory itself spans over a distance you would need days for, with an incredible infrastructure. Yet they can't stop the fishmen.
Also, why did Jinbe arrive so late? Big Mom had time to rest in WCI and read the paper, then arrive at Wano, wandering to Udon and fighting queen, then being brought to Onigashima to fight Kaido for days. What was Jinbe doing all the time?