I can imagine the reason they removed overworld pking is to allow pking to be somewhat competitive and lean slightly more towards skill than activity. By removing it, and by adding a new tower system (albeit flawed) they have increased activity in towers which is both good for the PK community and the towering community.
I understand people wanting to be able to kill passers by etc and that's fine, but let's be honest overworld pk just gets exploited by boosters and would just mean that towers are dead again, the top 50 each season would be full of boosters as was the case when they first introduced seasonal leaderboards. If you want pks go to a tower and get them. You shouldnt be able to just stand in one spot and farm them.
It's also good for the towering community, as it means tower guilds dont have as easy a ride throughout the season if they are constantly getting pked/attacked.