Why not throw everything into; tanks, artillery, ammo. men, advisors... Post WW2 they were everywhere, SE Asia, Cuba, Latin America, Africa, yet, the prize would have been Europe. Spain, a crown jewel and they let it slip away without a murmur.
When one thinks what if, one cannot help but wonder how much dif. Europe would have looked if the Communists of the USSR had fought the Nazis-Franco w/every means available. If world Communism was ever a goal it appears it was a great mistake on Stalin's part to so easily look the other way.Also, France near-by could have been a follow-up attempt with much sympathizers.
There are lots of dimensions to this question. Firstly, there's the simple fact that Stalin was not very interested in spreading world revolution because of the principle of Socialism in One Country. The failures of communist revolutions across Europe between 1917-1922 convinced Stalin of the futility of aiding in the subversion of European states towards communism. Stalin felt that it was more beneficial/sensible to construct socialism within the Soviet Union and shore up its own defenses against perceived aggression from capitalist powers.
Stalin's army was also not nearly as well-equipped as Hitler's or Mussolini's and therefore the assistance that Stalin could offer the Spanish Republic was limited. There were also additional diplomatic concerns- if Stalin offered overt military aid to Spain, this would anger the Western powers who, while not fans of Hitler, were equally distrustful of Stalin. Stalin understood that victory against Franco would need the help of Western allies like France and Britain. If he overplayed his hand in Spain, he risked alienating them. Conversely, he also had to worry that if he showed too much support for the Republic, Germany might also be compelled to invade the Soviet Union. Germany had signed the "Anti-Comintern Pact" with Japan and a number of European states, which sent a strong message to the Soviets that Hitler was prepared to undermine the Soviet regime by military efforts if necessary (which, as we know, he eventually did).
I think that we have to realize that Stalin had a myriad of priorities within the Soviet Union that prevented him from throwing full support behind the Republic. He was concerned about the preservation of his own regime and state, and to that end he felt that an excessive entanglement in foreign affairs could risk making too many enemies that might seek to undermine the Soviet Union. As long as he kept out of their affairs, or, as long as he offered limited support, he felt that he was playing enough of a diplomatic game of chess to keep different European powers satisfied.
If I'm correct the USSR under Stalin was state capitalist not communism, even though they might have projected that they were. I think it was during Lenin and the Bolsheviks that they were communist.
Stalin didn't throw everything into helping the revolutionaries because Germany and Italy were aiding the opposition. Stalin may of had the numbers but Germany's army at the time was far superior to his. It was not until Germany invaded the USSR that they started to produce better tanks and a more adequate Air Force.
Additionally, because their army was not as good if Stalin would have thrown everything to help the revolutionaries then it more than likely would have caused Germany to attack the USSR, something Stalin did not want at the time, and Stalin would of had a crippled army afterwards.
I believe this is at least a basis of why they didn't.