Why didn't the IGBT's output voltage change?
In second situation, we only change the pulse's voltage to 15V and the
other values same. However, I observed the same output voltage as 14V.

If the 10 volt gate signal is enough to adequately turn on and off the IGBT and deliver 14 volts p-p from a 15 volt power rail then, increasing the gate drive voltage isn't going to improve much on this situation.

What's the explanation of that? Why did the output voltage stay the
same?

The output p-p voltage can never exceed the power rail of 15 volts (unless you are using a transformer or an inductor which you aren't) so, given that the IGBT saturation level will be about 1 volt (when fully active), you won't get any significant improvement from 14 volts p-p on the output by raising the amplitude of the gate drive signal.