Why would they? They have more control of their own chain and they can select the BPs themselves and have almost the same security as EOS main chain and also have an ecosystem that is more aligned with Ono's specific application. Ono can also control the distribution, naturally. Lots of advantages. Same as Wax.
High value dapps might as well just run their own EOS chain with their own distribution. Low value dapps may skew more towards the main chain.
RE: ONO Will Launch the ONO Mainnet Using EOS.IO Platform