Why Starting and Running Torque of DC Motor > Than AC?

you said it,who told AC motors are maintenance free?

In industries, there are many DC motors used which can be replaced with AC motors .
Oh really? Where?
Most DC motors I see are small low voltage jobs, and I think you answered your own Q in 1.
If I want a nice safe low voltage motor with high torque for intermittent duty I'd use a brushed DC motor.
Del

1) why starting and running torque of DC motor > than simple AC motor?

The magnetic fields are synchronization locked (by physical layout).

In industries, there are many DC motors used which can be replaced with AC motors because of their maintenance as compare with AC motors. Why designers don't do that?

DC motors are basically traction motors and generally more robust than AC motors. If one requires high torque AC motors then SRIM is the option which would require six connections and two MTB's while a DC motor would have four connections and one MTB.

In general as I have observed in my steel plant DC motors last longer.