Why one salt favored over another?

disclaimer: nonchemist--ELI17

hey...this might not have a generalized answer, but I've sometimes wondered why with various compounds, final reaction to one salt is favored over another...eg, why do we see codeine phosphate in pharmaceutical preparations, which this counterion is rare in most other drugs? Why is amphetamine sulphate common, while meth is typically reacted to the hydrochloride? Why are research chemical tryptamines typically reacted to the fumarate? And why DXM HBr, particularly when it is not very water soluble?