Post History

I've just come across the following in Ernest Hutcheson's _The Literature of the Piano_, and it's the closest thing to an answer I've found.

 > In truth he [Haydn] was a greater originator than either Mozart or Beethoven; he created the sonata form, they adopted it, expanded it, and experimented with it. It may be thought extravagant to claim the creation of an art form for any one individual, yet in the case of Haydn the assertion seems irrefutable, for it is impossible to discover in earlier writers (including Carl Philipp Emanuel Bach) a true parallel to the structure of his symphonies, sonatas, and quartets.
 > 
Based on this, "father of the sonata form" might be more precise, and even at that I'd say that he brought the form to maturity rather than creating it. But this view of Haydn is, I think, what lies behind calling him the father of the symphony.