This is a question that I have heard different answers for in different places on the web, but never was quite sure what the exact reason was, but I have noticed that there is a group who believes that Carbidopa itself can be a problem and would prefer alternatives to Carbidopa while some people use an active component from green tea called EGCG as a substitute for the Carbidopa and especially if they are already using Mucuna Pruriens in one form or another.
I found one short answer to the above question that is the title of this post and here is a link to that very brief answer :
This is just one answer and may not be the only reason, but it is clearly one reason for the addition of Carbidopa to Levodopa that I am hoping to clarify for those who are wondering.
To add further to the general topic of L-Dopa / Sinemet/ Sinemet CR / Rytary, etc. here is another link to that subject which expands on that specific topic and answers some important questions that I have seen about the use of Sinemet on this forum :