Op Amp Output Voltage with Hysteresis
The simple answer here is that in this case the supplied Vcc sets the maximum output swing of the opamps. Per the UA741CP spec sheet a Vcc of +-15v gives a typical maximum output swing of +-13v (or 26Vpp).  

Since the UA741CP is not a rail to rail output opamp the saturation voltages of the output stage prevent seeing the full Vcc (+-15v) at the output.

For reference here is the AU741CP datasheet:
https://www.jameco.com/Jameco/Products/ProdDS/830522.pdf 
(See specification Vom).