Because of two pieces of information about silver.
1. Ag+ reacts with Cl- to produce AgCl, an insoluble ppt.
2. AgCl reacts with excess Cl- in high concns of Cl- (as is the case with concd HCl) to form AgCl2^- and AgCl3^2-, both of which are complex ions that increases the solubility of AgCl.
Why wouldn't HCl be a good acid to use to determine the solubility of a silver precipitate in acid solution?
2 answers
Perhaps I should mention, too, that the Cl- in HCl acts as a common ion to AgCl.