Agreed, PTI didn't do much for Karachi. But they are the only ones who CAN do something. No one else.
The current status quo is set up to suck Karachi's blood forever. The only chance Karachi has to get rid of the status quo is if PTI is in power with a strong federal government that can overturn the 18th Amendment and allow strong federal and local governments.
In addition, only PTI has a chance of taking away the Sindh government from the Zardari PPP waderas. With a new census and new provincial assembly constituencies PTI can win a majority in the Sindh assembly.
So, the choice is between a hopeless situation with the status quo, or lots of hope if PTI wins.