We need the ability to re-use global variables at different points in the topic. For example, if a topic asks the user to enter an email address, then checks AD to see if it exists, it might come back and ask the user to re-enter the email. Using global variables you end up with bot.Email and bot.Email1. So if a later topic wants to call this variable, it doesn't know whether to call bot.Email or bot.Email1.
This can be worked around using environment variables. I will make a blog post about this.