Why is there a comparative lack of variety in positive counterions in drugs?
Here is what I managed to find:



In various researches, cationic counterions were associated with a high prevalence of unexpected incompatibilities in drug development processes. Owing to their low solubilities, (typically  &lt;0.1% ) cationic counterions are infrequently used in many pharmaceutical preparations, however may be preferred to provide prolonged slow absorption while achieving effective plasma concentrations for some therapeutic uses, which is the case for benzathine and procaine penicillin G intramuscular injections.1 Furthermore cationic counterions, to a greater extent are susceptible to salting. Salting out results when highly hydrated inorganic ions (e.g $\ce{K+}$, $\ce{Na+}$) deprive organic ions and molecules of adequate water molecules to remain dissolved.

In addition another important factor influencing pharmaceutical salt selection is compatibility with excipients (diluents, binders, disintergrants, glidants, preservatives etc). Most amine derived counterions are incompatible with excipients. For example, the counterion of tromethamine (a primary amine) should not be used with reducing sugars as excipients because of a possible Maillard reaction, consequently limiting the available entities in the cationic counterions' pool .2 

The trend can also be explained as a consequence of the changes in research techniques employed by the pharmaceutical industry. It can also be attributed to small absolute numbers of approved drug products containing salts formed from acidic entities.3

Overally, anionic counterions are favoured in drug development due to the wide availability of  weakly  basic pharmaceutically  active principles (approximately 75% of drugs are estimated to be weak bases) 4, consequently leading to correspondingly low availability of cationic counterions for acidic drugs which comprise only about 20%, with remaining percentage being neutral. 



References


Newton D, W. Drug Incompatibility chemistry (American Journal of Health-System Pharmacy) Available from: http://www.medscape.com/viewarticle/590261. 
Wu, Y., Levons, J., Narang, A. S., Raghavan, K., &amp; Rao, V. M. (2011). Reactive impurities in excipients: Profiling, identification and mitigation of drug-excipient incompatibility. AAPS PharmSciTech, 12(4), 1248-1263.
Paulekuhn, G. S., Dressman, J. B., &amp; Saal, C. (2007). Trends in active pharmaceutical ingredient salt selection based on analysis of the orange book database. Journal of Medicinal Chemistry, 50(26), 6665-6672.
Manallack, D. T. (2007). The pKa Distribution of Drugs: Application to Drug Discovery. Perspectives in Medicinal Chemistry 1( )25–38.