Because local government can only be contested by a strong organization and every body knows PTI has no organization every body is leader in this party if someone get ticket the other don't wait and contest an independent candidate still it doing well
Perhaps you are unaware that PPP, JUI-f and ANP made an unnatural alliance(meaning they are left and right wing parties who always have different ideology) Vs PTI on 100% seats. So it was 3 parties vs PTI on all the seats, Yet PTI beat the 3 parties in majority of the places. Their seat distribution was like ANP(34% seats), PPP and JUI-F on 33% seats each.
If these Parties had contested elections in individual status, then these parties may not have got more than 30 seats each(currently they won close to 100 seats).. And all those seats would have gone in PTI's bucket..
Also note that unlike the General Elections people tend to vote to the candidate they think is near them and easily accessible for their problems and so they tend to vote for their neighbor than voting to a person in the far village.. While rigging in some seats can also win the corrupt people their seat..