Czechs and Slovaks didn't get their own separate nations because Czech and Slovak elite didn't even advocate for separate nations at Versailles. They advocated for Czechoslovakia.

Czech and Slovak nationalists elites advocated the idea of Czechoslovakism, idea that Czechs and Slovaks are one nation. While Czechs and Slovaks are somewhat culturally related, this move was 90% purely for political and practical reasons.

Austria Hungary, also nicknamed called Dungeon of Nations by Czech historiography, consisted of 2 dominant nations - Germans, Hungarian and many smaller ones under rule of those 2.

In Czech lands, Germans have been dominant in many aspect of society, and there has been historical attempts at Germanization of Czech society. Similar situation was in Slovakia with Hungarians.

Czech and Slovak elites knew that alone they were very weak and therefore united in a struggle for independence and started marketing the idea of Czechoslovakism. They knew that Great Powers which would be deciding future of Europe after WW1 would never accept creation of such small nations as Czechia and Slovakia.

There was also a very politically awkward problem that in Czech and Slovak lands there were substantial German and Hungarian minorities. In fact they were so substantial that in Czechoslovakia there were more ethnic Germans than Slovaks. This problem was solved by idea of Czechoslovakism. Germans no longer outnumbered Slovaks, since they were Czechoslovaks.

You may be interested in what /u/veercingetorix wrote about the breakup of Czechoslovakia in 1993, especially the 2nd paragraph.