Why do people recommend cream of tartar when whipping egg whites to the exclusion of all 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.