Retrospectively, the only answer to your question is: "because of poor grasp of the local political and cultural context".
In the 1920s, dominant countries were organized on the notion of nation state. This was a relatively recent development; for instance, Germany had formally existed only for 50 years or so at that time. Other countries had turned into nations somewhat earlier; e.g. France converted from loose feudalism to nation over about five centuries (from the first attempts at bureaucratic centralisation under Philippe le Bel to Napoleon's nation-building epic). Crucially, some peoples had not, at that time, completed or even began their nation-creation process. This is in particular true of areas which had long been part of empires, in this case Ottoman and Austro-Hungarian.
To a large extent, a nation can be created only through events which define it in the psyche of its individuals; such events must impact all or most of the future nation's population, but exclusive of others; that exclusivity is what makes the nation. In the context of empires (which were the norm throughout history, before modern times, for large scale political constructions -- read John Darwin on that subject), nations thus rise through resistance to a conquering empire, or emergence of a local leader amid the chaos of a disintegrating empire. There is no sure-fire method for creating a nation, but it seems reasonable to state that nations could not develop and flourish while within the boundaries of functioning empires. The decline of the Ottoman empire had allowed proto-nations to begin to form but (as yet) this process was not complete.
Yugoslavia was the result of the unification of several provinces taken from both Ottoman and Austro-Hungarian empires, a concept which was based on the (flawed) ideas that:
- Nations are large and must cover a "decent" area and population. No micro-state !
- Nations correspond to the then-popular notions of "races" which can be approached through linguistics.
In that sense, the WWI victors pushed for the creation of a single state of Yugoslavia: Serbia, Croatia, Montenegro... could not be nations on their own (too little), and there was no problem in putting all of them in a single country since they were all "South Slavs". This kind of myopia, when it comes to defining post-WWI borders, is famously illustrated by the 14 points of Woodrow Wilson, who, from Washington, could idealistically rant about the necessity of adjusting frontiers "along clearly recognizable lines of nationality."
As for Czechoslovakia, a similar process implied grouping areas based on language, but other forces were at work too. The Czechoslovak Legion fought along with the Allies so as to secure independence after the end of the War, with as big a territory as possible; e.g. inn 1919, Czechoslovakia and Poland fought a small war over some disputed areas.