Per andy8052 on the Uniswap forums: ...there is immense value in being available for swaps on every EVM compatible network. Since there is no liquidity mining, it is hard to make the case that multiple deployments will segment liquidity, and long term if this is an issue governance can flip the switch on liquidity incentives for the chains it desires deepest liquidity. In the meantime, not deploying to new and growing networks like Arbitrum and Matic just allows for other exchanges to come in and fill that spot. This could potentially eat into fees earned by governance in the future.
Alongside this, it will help to grow the potential list of projects that can be built on Uniswap v3. Growing the public smart contract libraries interfacing with and using the v3 codebase will only expedite Uniswap adoption on all chains as the code and integrations become more battle tested.
Arbitrum launches later this week on May 28th. I think it would be best to start a discussion ASAP around deploying the Uniswap v3 code to Arbitrum network.
Indicate your interest for Uniswap moving forward with a deployment of Uniswap v3 on Arbitrum, by (1) opening the Uniswap v3 license to include code deployed on Arbitrum, (2) allowing community developers to implement Uniswap v3 on Arbitrum.