well, i'm not sure exactly why, but i would guess that it's because vitamin c becomes inactive when it's exposed to air. this is why juices are best when made fresh and drank right away. more vitamin content. also, organic lemons are the best that way you are not putting in the pesticides from the peel into your body.
when you cleanse you become very sensitive and don't want extra junk in your body.

First of all...Bottled Juice is dead.
It has no live enzymes in it because it has been heated so it can sit on a shelf.
You need live enzymes to clean out your cells.
Bottled lemon juice is mainly just a flavoring.

Secondly...buy or borrow (from a friend or relative) an electric juicer. I bought one from Walmart for $40.
I have tried hand juicing non-organic lemons and you are right...you won't make much juice per lemon.
Before it took about 3.5 pounds (15 medium-sized) of non-organic lemons to make 2 cups of juice.
Now that I'm juicing whole organic lemons, it takes 5 to make 2 cups. (The lemons here in Texas can get to tennis ball size.)
The juice is foamier and does have a little of the peel in it but the Master-Cleanse suggests taking some peel to move bowels anyway.
And if you can believe it, the national chain grocery store down here sells the organic lemons for $2.29 a pound and the non-organic ones for $2.49!
I wish I would have whole juiced organic lemons from the beginning, 30 days ago. I probably would have saved some $$$$.

So if you use bottled juice, you are not helping your body rid itself of toxins and good results from all your effort will take much longer. And who wants to be on this fast any longer than necessary?
I hope this helps.