The demand for BSC network has increased due to increase in gas fee of Ethereum network. Most of the Altcoin projects are now using the BSC network. Even some Ethereum network's Altcoin project is switching to BSC / Polygon network.
But now it is seen that some Altcoin project is being launched in Polygon network. We know that gas fee of BSC network is very low and transaction speed is high. So for which benefit, all these projects are being launched on Polygon network instead of BSC?
The answer to your question is very simple; the same reason why a lot of projects are leveraging BSC network because of high gas fees in Ethereum, is the same reason a lot projects are leveraging Polygon network now because of the increasing transaction fee of BSC network. BSC network gas fee is gradually increasing and congestion gradually coming up as well, and since Polygon offers a more cheaper transactions fee with better speed and currently no congestion, a lot of projects are moving to it. And as time goes on, a tougher competition will grow up between BSC and Polygon, at the same time, if Polygon team gets all their works and features right, the same attention and growth which BSC network witnessed and enjoyed months ago might take place within Polygon (but this is just my speculation as other blockchain networks are not relenting like Fantom).
Lastly, most projects are also migrating to different blockchains without any tangible reason other than to pump their token price.