Toons can't talk in the our world because the transition from the toon world to our world really tickles. So much so that if they try to talk all they will do is laugh until they get sick. It has been well documented in the Toonland Medical Journal over the years, they call it von Toon Transvarlaughter Symdrome (VTTS, pronounced "vits" with a german accent). I believe is was named after Professor von Duck who Walt assigened to study this issuse of toon transfer prior to the opening of Disneyland.
VTTS was discovered the first time Von Duck transfered between toon and real world. It was re-affirmed in von Duck's following one thousand and three transfers. Professor von Duck finally figured out that by keeping his mouth closed he could control the laughter and therefore not get sick...or as Von Duck said " By keeping mine mouth shut I vould not toss mine cookies" (1).
Roy, seening the mess that von Duck made at the Disney studios during the transfer experimentation phase, confronted Walt on the issue. Roy forced Walt to that toons are shall not be allowed to speak(2). Roy insisted that the rule be placed in the Disney Toon hand book because he was worried about a lot of guest lawsuits that could have ruined the new park's bottom line.
Walt had to sell this issue to the the tough American Toon Labor boss but in the end Goofy agreed to rule with out too many concessions. One of the most important aspects of the March 1955 ATL agreement was that all toons would be allowed to appear in public at twice their normal toon world size. Walt was worried about guest reaction to that size but with park opening looming he agreed.
Hope this helps
1. American Toon Medical Journal, May 1953, pg 15
2. The Walt Disney Company Toon Cast Member Handbook
7th edition, December 2004, pg 204