A few reasons:

• In 1the 19th Century there was an "Austroslavic" movement to protect Slavic interests against Austrian (German) interests. Both Slovaks and Czechs (and some Yugoslavs) were in this movement together.
• At the outbreak of WWI It wasn't just Masaryk (Czech) but Slovaks like Rastislav Štefánik that were pushing for independence of a unified Czechoslovakia.
• Czechs had a huge industrial base, while Slovaks were much more rural and agrarian. I think Slovaks realized they would be better off with the much wealthier Czechs. Also Czechs had been independent 300 years before. For Slovaks it was more like 1000 years. Slovakia just didn't have as strong of a claim for independence than they did together as Czechoslovakia.
• In the Treaty of Saint-Germain (sort of like the Versailles treaty, but for Austria-Hungary instead of Germany) when Hungary was given independence and the real union with Austria terminated. This treaty also had a certain philosophy about it to end majority rule in the former Empire. The way they did this was to break up the borders among ethnic lines. It wasn't perfect, but Polish parts were given back to Poland. German speaking parts of Hungary were added to Austria, Bukovina went to Romania, so on and so on... and Czechoslovakia became independent. Austrians and Hungarians both had been pretty harsh at times to minorities in their countries. The Allies saw this as a way to reduce the likelihood of further ethnic strife in the region. It was also (I would argue at least) punitive to Hungary.
• The majority of Slovaks themselves wanted this. The Slovak people also were far less supportive of the idea of independence in 1993 than the government was.

TL;DR Slovakia was extremely rural, while Czech had a large industrial base. Slovakia hadn't been independent for a millennium. Hungary reduced to 50% of it's formal size on all sides (not just Slovakia) by the treaty after WWI

This has been asked and answered several times. In addition to the answer by George_xvi, there was a fairly simple but significant reason for a single country and the promotion of a united nationality: to provide a sort of a counterweight against various sizeable minorities, particularly Germans and Hungarians, and a strong basis for an 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 Czechoslovak nationality is a somewhat artificial concept made prominent by the leaders of the independence movement, chiefly 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 – Sudetenland. In Slovakia, the concept of Czechoslovakism helped counter the Magyarization of the region, as 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 took 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.