Why do people recommend cream of tartar when whipping egg whites to the exclusion of all other acids? Virtually every single egg white recipe will say add some cream of tartar to egg whites to help them whip better because the cream of tartar will lower the pH and make it more stable. But if all it's doing is making it more acidic, why exclusively cream of tartar over the dozens of far more common acids found in the kitchen? 

One difference is cream of tartar is the only solid acid so it wouldn't add more water to the egg whites except many books also say that adding a tbsp of water to egg whites increases stability as well. Is there something else in cream of tartar that isn't in other acids?
Ingredient substitution lists say you can use an equal volume of lemon juice or vinegar if you don't have cream of tartar.  

Most likely, the assumption has been that a baker will be more likely to have cream of tartar on hand than other acid sources due to the fact that it has multiple uses in the kitchen:


Leavening
Stabilization of egg whites
Prevent crystallization of sugar in things like frostings, syrups, chocolates, etc.  


Cream of tartar also has a number of beneficial properties:


It is odorless and practically tasteless, unlike lemon juice or vinegar.
It acidifies without adding water, which might be detrimental in some applications.  
Unlike fresh lemons, cream of tartar has a nearly indefinite shelf life.