Polygon was the cheapest one. The problem is if you are minting/uploading 10k collection to the open sea which is a lot. This is also making the transaction fees are so high when you are ethereum and it may cost more than it. You can calculate it based on the average transaction but i think that when you wanna mint your own NFT and this will cost you a lot when you are using ethereum. You shall pick polygon to decrease the fees. this will make even more sense when you are uploading thousands of NFT.
So many people are using polygon right now caused by the cheapest fees