Because that first step of USD(cash) -> Swiss Franc has to go through a bank and/or Forex exchange which has to follow U.S. anti-money laundering regulations. If you attempt to place a large amount of cash into a bank or forex account you will have a lot of paperwork to fill out to reasonably prove that it isn't illegitimately gained money.
Whether those regulations are actually effective is questionable (see HSBC, BoA, etc.), but that's the rationale.
I'm not really familiar with Mt. gox, but I'd assume you can send in a money order or perhaps even cash. I don't know anything about Dwolla either, maybe they have a method of bypassing banks? I might be wrong so if someone knows better please correct me.
Edit: Did a little bit of digging[1], looks like they accept wire transfers (e.g. Western Union), and I'm sure there would be ways through those e-currency and other services to deposit large amounts of unregulated money.
Whether those regulations are actually effective is questionable (see HSBC, BoA, etc.), but that's the rationale.