I think the X-axis only shows the SNR for the case "bandwidth = bit rate". The other curve "bandwidth = 1.5 bit rate" can only be used to see the change in error probability when the IF BW is changed.
The SNR will also change, but that value can't be read from the diagram. We can draw the conclusion that the SNR gets worse, since the error probability increases.
To really understand the diagram I think we also need a diagram that plots the SNR as a function of the IF BW. I expect a peak not far from BW = bit rate.
If the wanted signal already "fits" inside the BW, nothing is gained by increasing the BW. The added signal will mostly be noise, so the SNR will be worse.