Why does Carnot Engine have the curves as isothermal and adiabatic? Why exactly does the Carnot engine have two isothermal and two adiabatic curves? (My book says that “a process is quasi-static if the temperature difference between the system and the reservoir is infinitesimal. Hence it is isothermal. For the connecting two isotherms ;only reversible adiabatic is suitable. If(say) it were isochoric then we shall need a series of reservoirs in the temperature range defined by the two reservoir and the sink” .I don’t get it. Please help.
By definition, a Carnot cycle is comprised of 2 reversible isotherms and 2 reversible adiabats.

Regarding your question about isochoric heating or cooling, if you are going to carry this out reversibly, then you can't do it with just one constant temperature reservoir.  For the heating or cooling to be reversible, the temperature difference between the system and the reservoir(s) must be tiny over the entire path.  To do this, you need a sequence of constant temperature reservoirs running from the starting temperature to the ending temperature.  After the system has equilibrated with each reservoir in the sequence, you move on to the next reservoir.  That way, the temperature difference between the system and the reservoir is tiny along each step of the process path.