Why was Tito able to rally the loyalty of so much of the young people in ex-Yugoslavia, and the royalist Draza Michailovich (Драгољуб "Дража" Михаиловић) was not? The so-called chetnicks, loyal to the last Yugoslav king, were in much less numbers than the partisans.
After reading up on the Chetnicks during WWII. Wow, they were a nasty piece of work.
So lets look at the two groups here. On the one hand we have Tito's partisans. They are communist, one of the popular up-and-comming ideologies of the early 20th century among idealists (particular slavic ones, after the Russian Revolution). They are fighting to free their whole country, Yugoslavia, from foriegn invasion.
Then you have the Chetnicks. They are in modern terms sort of a cross between modern neo-Nazi nationalists and Hezbola. They are fighting against anyone who doesn't look like them (Muslims, Croats), or doesn't agree with them (Communists, Axis occupiers). After a while, they even stopped fighting the Axis, arguing that the Communists were a bigger threat. So they essentially became collaborators.
Now personally I never understood the pull of racist right-wing ideology. So I don't see why any young person would have joined the Chetnicks at all. But let's instead be objective about this and try to take stock. To be a Chetnick, you'd need to be a right-wing racist and you'd need to be ethnically Serb. Based on the nearest census numbers (1921 and 1971), only about 40% of the population of the area was Serb.
Alternatively, if you happened to be a sociopath, the nearly indiscriminate terror and the impunity might appeal to you.
On the other hand, anybody who wanted to fight the occupiers could go join Tito. Thus one would expect Tito's followers to be more numerous based on the simple math of the situation.
USA National War College ascribes the cause as mastery of Engels, and through Engels a mastery of Clausewitzian warfare. I think that is a more than little dubious as it comes down to "Nothing succeeds like success."
I'd normally point to the spiral of violence type argument, which argues that revolutionary forces controlled the tempo of violence in a largely traditional society, causing the occupiers to inflict disproportionate violence on civil communities, thus legitimising the revolutionaries. Obviously, you need to do this "right" rather than just doing it.
Strangely enough, I believe that it was the ideology of Communism. That is, the various ethnic groups under Tito put aside their differences, because "we are all Communists here." (I have no sources on Yugoslav Communism, and am basing my answer on Chuikov's "the Battle For Stalingrad," in which "Communism" was a unifying force for the defenders. A similar factor was at work in Mao's China.)
The Chetniks didn't have any comparable unifying force. About their only ideology was Fascism. And the Fascist ideology of the time seemed to be best expressed as "Mein Land ist besser als dein Land." (My country/nationality/race is better than yours.)