Because it doesn't do anything, it requires banks to move client's hedging investments in certain swaps out of bank accounts and into broker dealer accounts, which really does nothing. Sure the bank accounts are FDIC insured and the broker dealer accounts aren't, but there's no scenario where the investment banks go down and the banks don't. Also, these types of derivatives (interest rate swaps, for example) had nothing to do with the financial crisis.
So there's what it doesn't do. What does it do? Well, for one it basically lumps a bunch of different types of derivatives together and treats them in the same manner, without understanding the risk profile of the very, very different types of swap and forward contracts that it regulates. It increases issues in the banking industry because banks can't easily determine what kind of "risky" derivatives fall under the rule. The market is very complex, and this regulation shows that the SEC doesn't understand that. Finally, the concept of "risk" in certain derivatives isn't uniform. For example, many of the "riskier" swaps are used by companies to hedge, which actually stabilizes the market.
Why do banks want the push out regulation revoked? Pretty simple. It increase costs and risk for the big banks that don't fully understand how to properly apply the regulation because the SEC themselves don't really know. It makes it more difficult to service these contracts, which aren't inherently risky so that isn't necessarily a good thing like most anti-bankster folks will claim. And so bank costs and difficulty of doing business go up, which translates into a lower level of service for their clients. This isn't a conspiracy to deregulate the financial markets, it's an attempt to revoke a very confusing and ignorant law that doesn't really do anything except make it tougher for banks to do business.