The difference is that ETH1 will bridge to ETH2 in the future, and therefore by choosing ETH2 as your substrate, you’re predicting that transaction costs for sibling-transactions crossing from your chain to ETH1 will become cheaper when that happens.
A similar promise cannot be made for any other substrate network, as ETH1 isn’t going to bridge to any other substrate network.
Since it’s not like sub-chain mainnets can pack up and move house after they’re launched, when launching one, you have to make choices based on how things will be, rather than how things are. It’s sort of like choosing a city to build a corporate headquarters in, based on what the civic infrastructure is likely to look like in 20 years.
A similar promise cannot be made for any other substrate network, as ETH1 isn’t going to bridge to any other substrate network.
Since it’s not like sub-chain mainnets can pack up and move house after they’re launched, when launching one, you have to make choices based on how things will be, rather than how things are. It’s sort of like choosing a city to build a corporate headquarters in, based on what the civic infrastructure is likely to look like in 20 years.