Originally posted by Steve.Ledger Because you get two fights for the price of one.
These comparison threads are a little tiresome since the specs for all models currently available are nicely summarized and published on this site using the comparator.
https://www.pentaxforums.com/forums/pentax-cameras-compared/?c1=k3&c2=k5ii&c3=k50
I would much prefer questions specific to the potential buyer's use cases. In the case of these two cameras, both are incredible values currently on the N. America market and both are very desirable cameras. The points of distinction are obvious from the specs, though the applicability to the OP's usage might not be. The K-50 will do an admirable job of documenting with the live-view focus peaking being a prime feature.
Steve