Modified from an earlier answer

The seemingly unlikely emergence of a Czechoslovakia is largely the byproduct of diligent Czech nationalist activism, Allied postwar imperatives, and the sudden nature of Austro-Hungarian collapse. Of the Dual Monarchy's national minorities, the Czechs were in an advantageous political position for the chaos of World War I. In the prewar period, the Czechs were able to develop a relatively detailed political program and assert some degree of national rights versus the German minority. The various prewar fights over the national question meant that the Czechs had a highly developed political discourse about national rights and the geographic contours of their homeland. Czech national activists were able to posit that the Czech nation had a long historical pedigree (Bohemia) and paint other nationalities as either benign or pernicious to a future Czech state. The various writings of Masaryk such as Nová Evropa and those of other Czech intellectuals convinced the Allies that the Czechs had a clearer understanding of the nationalities issues than other groups. The activities of the Czech Legion and Masaryk's contributions to Allied intelligence also added a greater gloss to the Czechs that placed them in higher esteem than other groups. In particular, Masaryk and the Czech exile community were able to paint the Magyars as co-conspirators with the Austro-Germans in repressing national minorities. This contributed to a perception among the Allies that the Slovaks would be better off in a union with the new Czech state. The Czech exiles argued they could integrate the Slovaks far better than either the Poles or the Hungarians. In light of Wilson's call for national self-determination, Czech nationalists contended that the mutually intelligible nature of Czech and Slovak made the two nationalities a natural fit together with the Czechs assuming the senior role as the most developed nationality. There was no real Slovak voice in these matters.

An enlarged Czech state was not an automatic given in the immediate postwar. Although the Allies favored an enlarged Czech state, the near total collapse of Austria-Hungary created a conditions that could undo all the plans of the Czech exiles. Fortunately for the Czech movement, the political leaders were able to swiftly take over the intact administrative and military apparatus of the dissolving Austro-Hungarian state. This was where the chronic ineptitude and weak state of the Habsburgs played into the Czech hands; there was no supranational framework to resist this quiet Czech administrative revolution after the abdication of the Habsburgs. The Allies welcomed this development as Czech state was relatively stable and would be a lynchpin of the "little Entente" France was seeking to construct within Eastern Europe. The Slovak nationalists had no practical alternative but to be a part of the new state. The Czechs promised a degree of national unity and stability after a tumultuous war which also added to their attractiveness of accepting Czechoslovakia.

The Slovak nationalist activists had tended to make common cause with the Czech nationalists operating abroad during the war. In hindsight, they had very little choice in their ally. This was a function of the fact that the Magyars under the Dual Monarchy tended to try and push for the one-nation Hungarian state which entailed Magyarization. An alliance with the Czechs was for some Slovak nationalists was the least worst alternative. The disorder in Hungary following Béla Kun's declaration of a Soviet republic and the Romanian invasion made an accommodation with the Czechs more appealing. Jozef Tiso, the head of the Slovak People's Party, tacked towards a policy of pragmatic cooperation with the Czechs with the long term-goal of Slovak autonomy. Although Tiso cooperated with Prague, he always made sure to assert that autonomy was the ultimate goal of the Slovak national movement. Both Masaryk and Benes were pragmatic when facing Slovak demands for autonomy and granted modest concession in the 1920s in exchange for the Slovak People's Party joining the government.

Sources

Hilde, P. S. "Slovak Nationalism and the Break-Up of Czechoslovakia". Europe Asia Studies. 51, no. 4(1999): 647-666.

Zückert, Martin. "National Concepts of Freedom and Government Pacification Policies: The Case of Czechoslovakia in the Transitional Period After 1918". Contemporary European History. 17 (2008): 325-344.

The following is my response to this question posted here some time ago – there was a fairly simple but significant reason for the creation of a single country of Czechoslovakia and the promotion of a united nationality: to provide a sort of a counterweight against various numerous minorities, particularly Germans and Hungarians, and to form a strong basis for the newly independent state after the fall of the Habsburg monarchy.

Czechs and Slovaks have mutually intelligible languages, are close culturally and have lived in close proximity for centuries. The earliest proponents of Czechoslovakism in the late 19th century have also pointed out historical ties such as the realm of Great Moravia which was indeed populated by various peoples including the two in question.

However, the idea of a single Czechoslovak nationality or identity is a somewhat artificial concept made prominent by the leaders of the independence movement, chiefly Czech elites such as future presidents Masaryk and Benes, before and during the Great War for the reasons outlined above. The readily compatible nature of these ethnicities made it easy for an idea of a single nationality to be officially adopted, especially in light of the diverse population of the newly independent country.

According to the 1921 census, there were 8 759 701 Czechoslovaks (roughly two million of which were Slovaks) and 3 123 305 Germans in Czechoslovakia. This was a direct result of Czechoslovakia keeping its historical borders which included regions with a significant German population – the so-called Sudetenland which would go on to become a source of great tension between Czechoslovakia and Germany. In Slovakia, the concept of Czechoslovakism was introduced as a counter the Magyarization of the region, since the Hungarians were in an even stronger position than Germans in Bohemia and Moravia when compared to the local populace.

The idea of the two peoples being a single nation was not without its opponents, however, and while it did take hold and was in fact an ideology enshrined in the constitution, in the end it was Czechs and Slovaks themselves who abandoned it. After the communists came into power in 1948, the official stance turned away from the single nationality concept and in 1969 the Czechoslovak Socialist Republic was split into a federated state of the Czech and Slovak Socialist Republics, and in 1993, the federation would dissolve for good.