Why is it more important to monitor your word choice in a professional environment than in personal communication exchanges?

It's more important because you could say something that can offend someone and you don't find it offensive.
 Using the wrong word in a professional environment can cause repercussions throughout your organization, whereas personal is less formal. 
The main reason is because of audience and the impact it could have on where you work or with the people you work with 

It is less likely that the shoe will slip while someone is walking