I suspect it is more to compete head-to-head with Westjet. Westjet does have non-stop YVR-PSP but not at the frequencies of YYC-PSP, so more than half-the-time Westjet is YVR-YYC-PSP. Although, right now YVR has more non-stop choices to PSP than YYC.
WS operates YVR-PSP seasonally (4x weekly). I'm actually surprised that there wasn't an announcement of increased frequency for this winter because we were filling our -800 almost daily. Hopefully we'll see an increase in our winter schedule when our enhancements are announced.