I am finding it difficult to understand how they did not feel a mandate to destroy totalitarianism in Europe (at least Western Europe) after Germany conceded

The war in Europe wasn't about destroying totalitarianism. There was a good deal of propaganda to that effect, but the UK went to war over Polish neutrality and Germany declared war on the US shortly after Pearl Harbor. The western Allies also saw a showdown with communism on the horizon. That is where Franco had his opportunity to stay in power. Franco may have been a murderous authoritarian, but he was also vehemently anti-communist. With many other European nations appearing to wobble or even lean towards communism (Greece and Italy to name only two), having Spain locked up meant one fewer place the US and UK had to worry about.

Also, the defeat of Japan was not yet complete. Convincing US and UK citizens that instigating another war in a mountainous nation when there had already been so many casualties in WWII would not have been a trivial matter.

As always, followup questions from OP and others are encouraged.

The first post here is correct - the Allies were not interested in fighting another nation that they really had no business fighting in the first place. Franco was on good terms with Hitler, but he refused to go to war unless his demands were met (which were comically outrageous in Hitler's eyes). This probably saved him from Allied invasion.

Once the war was over, Franco's anti-communism got him some real friends and funds in the west.