We generally believe that the fee-tier is one of the main reasons. There is certainly an impact from liquidity differences, but it should generally be negligible for the vast majority of retail traders.
Because retail flow generally has such small price impact, liquidity differences do not matter more than the difference in the fee. You can see this in Table 1 from our previous written papers on the costs of swapping on AMMs. While the data is from Ethereum Mainnet, it should be a decent data point.
According to the paper, the average medium-sized ($1k - 100k) ETH/USDC retail trade has a price impact of .7 bps. Since the fee difference is greater than this amount, no amount of liquidity differences for the median trade will bring back flow.
To bring back flow without changing the fee, the difference in price impact would have to be larger than the differences in fee. The fee costs is fixed, while the price impact is based on the size of the trade, meaning that only larger trades would be eligible for routing back to the Uniswap Protocol with increased liquidity.