People do flyer campaigns all the time for other things. Mailing is going to cost like 50x as much as just printing a flyer, and to even do bulk mailing you'd be required to get a permit which I mean come on...

They do other things too, but flyer campaigns were effective, cheap, and low risk. Other methods of spreading a method require a lot more time, work and risk to reach far fewer people. The only reason the law exists is to harm GDL's ability to use free speech effectively.