First, I have to caution you on describing Franco's Spain as fascist. Its applicability would depend greatly on which definition of fascist you use. Given Franco's origins in the establishment as a military officer as well as his strong ties to monarchist movements such as the Carlists, his categorization as a fascist is problematic. Further, the Falange (the Spanish fascist movement) garnered abysmal results in the 1936 elections just prior to the Spanish Civil War.

As to why Spain didn't join WWII, the response by /u/Georgy_K_Zhukov is the best short explanation in my opinion. I encourage you to read it, but basically Spain was exhausted after its civil war (which ended in 1939 before WWII began in Europe) and never was convinced to join the Axis but did supply troops for a division on the Eastern front for a time.