Was there a reason? Just another anime question...
Since I trust Blobyolo, I will change my comment into an answer.
No one really knows. Whenever they mention Prof. Ivy, Brock becomes depressed and curls into a ball and says "Don't mention that name".
This might help:http://bulbapedia.bulbagarden.net/wiki/Professor_Ivy
And maybe this:http://www.google.com/search?client=safari&rls=en&q=Prof.Ivy&ie=UTF-8&oe=UTF-8
P.S. I know the one above is google, but many people have asked the same question. Check out a couple websites and maybe that will answer your question a bit more.
I hoped this helped!