Agreed. Maybe they thought there is risk Murray actually bounces back and gives them good stretches of goaltending, while Mrazek is just pure trash and helps more with their obvious run for Bedard.
Matt Murray is actually still good when healthy (which is 50% of the time at best)
How do I know that? Well, outside of the eye test, he had a 0.600 QS% last season (quality start percentage). Carey Price had 0.591 the year he won the Vezina, Hart, etc.
Mrazek had 0.333 SV% last season
So yes, if (by miracle) Murray was able to stay healthy a full season, he could actually post a ~0.920 SV% and ruin a tanking effort. That would have been his approx. SV% last season without the outlier games vs Toronto on January 1st (Ottawa Zombies) and vs Arizona Gretzky impersonation from Schmaltz