Because Polygon is a different chain and people have billions locked in Ethereum. Layer-2 allow users to transfer their tokens to the Layer-2 without trusting a third-party bridge. Funds on L2 are secured by L1.
I can perhaps shed some light on this. I am the founder of Moonstream (https://moonstream.to). Our customers use our API to consume on-chain events from their off-chain infrastructure. We currently support Ethereum and Polygon, and run our own nodes on both blockchains.
Polygon is much less decentralized than Ethereum. There are 100 validators on the network. In the future, there will be some sort of scoring and auction mechanism to ensure only healthy validators. Currently, this mechanism has not been deployed. All 100 slots are taken by an in-group of node runners. It is not a pleasant experience to run a node on Polygon and not be in this in-group because the team does not communicate well with anyone outside the in-group. For example, in December, we experienced several days of downtime after the Polygon team made an emergency security patch that took down almost all nodes on the network but only communicated it to the in-group. This outage hit us, Polygonscan, and many independent node operators and was not a pleasant experience. The Ethereum team, on the other hand, is excellent about communicating upcoming changes, even emergency patches. Things really feel much more open and accessible as a node operator on Ethereum than they do on Polygon.
It is hard to transfer value to Polygon. Most exchanges do not support moving value directly to Polygon (Binance is the notable exception). Most use the Polygon bridge (https://wallet.polygon.technology/bridge). Using the bridge to move from Ethereum to Polygon requires submitting a transaction against the bridging contract on the Ethereum mainnet and current gas costs make this very expensive relative to the value most people want to transfer. You pay roughly $100 for any transfer of value, but most people are transferring less than $10,000 in value per transaction. Also, this bridge went down for over 24 hours last week when Polygon released EIP1559 support. I believe this is a major obstacle to Polygon adoption.
Finally, as a developer, Polygon frequently experiences deep reorgs - up to 40 or 50 blocks deep in my experience. This makes it really difficult for conventional servers to work with on-chain state. Ethereum is really much more stable as a blockchain.
Anyway, I'm not really shitting on Polygon. We do deploy all of our own smart contracts to Polygon before Ethereum because it is a production environment and allows us to incubate our on-chain features with real customers and real feedback before we take them to the show. It also gives us ample time to figure out where we need to optimize gas before moving to Ethereum.
The only part that pisses me off is the in-group/out-group dynamics around running a node, but mafias exist everywhere and no blockchain will change that.