I'm sorry, but the question is a little unclear. Do you mean intervene against him during the Spanish Civil War (1936-1939)? During WWII? After WWII?

During the Civil War:

Regardless of your position on militarism at the time, it simply wasn't advantageous for anyone to get involved in Spain. If you were a pacifist, you'd oppose military intervention. If you were pro-military, you'd be much more focused on re-arming your own country than throwing away valuable weaponry: the latter was a significant reason given for the British joining the Non-Intervention Committee and refusing sales to the 2nd Spanish Republic.

During WW2:

Why would you open up a second front to fight a non-aggressive nation? After failing to help Hitler in taking Gibraltar, Spain did very little to aid the Axis powers, and so wasn't a target. Attacking Spain would only draw troops away from either Italy or France who were sorely needed.

Post WW2:

Franco tried to portray himself as primarily as anti-communist rather than fascist (which didn't fully define the Nationalists anyway). At this point, you may as well turn the question into 'Why did the West support anti-communist dictators during the Cold War?', which I'm sure someone else could answer better than me.

Sources:

Antony Beevor's The Battle for Spain

Gerald Howson's Arms for Spain

Paul Preston's The Spanish Civil War