Liquidity rules are the main reason. Even Berkshire Hathaway had to give in and create the Class B shares in response to pressure from institutional investors constrained by liquidity requirements. Brokerages are also less willing to allow margin trading on equities with low liquidity because of the risk that a customer will be unable to sell their assets to cover margin calls.

As for fractional shares, keep in mind that these are a creation of brokerages, they are not universally available, and they create complications with shareholder rights (e.g. voting).