Unfortunately not all states have voter referendums as part of their state constitution, WI being one of those. Its especially dissapointing because in a poll, 80% of WI residents were for medical marijuana usage.

Wisconsin Poll Shows Support for Medical Marijuana
Wisconsin Poll Shows Support for Medical Marijuana. Is My Medicine Legal Yet. June 2002.

A public opinion poll commissioned by Wisconsin-based Is My Medicine Legal Yet (IMMLY) asked the question, "Would you support or oppose the Wisconsin State Legislature passing a law to allow seriously ill or terminally ill patients to use marijuana for medical purposes if supported by their physician?" The poll found that statewide, 80.3 of respondents answered yes, 16% answered no, and 3.7% don't know/refused.
Its the Republicans in the state senate that are preventing this from happening, the best thing we can do is try and get those dirty elephants out, which thankfully will probably be happening quite soon with the current political climate.