An old question, but why wouldn't I answer. "Yugo" in Yugoslavia comes from South in South Slavic languages, Czechs and Slovaks are Western Slavs like Poles and Sorbs (Slavs living in eastern Germany). Yugoslavism has concerned only Southern Slavs, logically.

It would also be hard to build a nation with almost nothing connecting it. South Slavs are the weird ones out culturally and West slavs linguistically. Generally speaking Polish, Czech and Slovak are mutually understandable, albeit rather difficult to get the exact meaning of and "funny" South Slavic languages are basically identical and hardly understandable for Western Slavs

Also take into consideration lack of border between Czechoslovakia and Yugoslavia

Czechs also straight up weren't interested in being the non-dominant group in their country and Slovaks wanted to preserve their culture and stop magyarization and as their culture was probably the closest to Czech one they decided to stick with Czechs

Czechs and Slovaks were included in pan-slavic agenda, also look up Polish-Czechoslovakian confederation (a Polish proposal of unifying Czechoslovakia and Poland proposed in the interwar period and worked on until early post-war period which was ultimately abandoned due to the Soviet Union being against it)