Why there was only one episode of Black Clover?
Your premise of the question is wrong! 

The animated episode is NOT a series, but rather a single "Special Episode" bundled with the 11th Volume of the Manga. This is mentioned as much in MAL page for the episode. Afaik, it adapts the first chapter of the Manga. [Confirmed in Comments]

https://myanimelist.net/anime/33950/Black_Clover


  The Episode "Black Clover" was screened at Jump Festa 2016 on November 27, 2016. The OVA was later bundled with the 11th volume of the manga, released May 2, 2017


In December 2016, there were talks about a TV adaptation of this Anime slated for release in 2017, however since only the OVA has been released, maybe they are testing the waters with the OVA release. The full series adaptation has "no official date yet"
Here is the ANN page for the same - Black Clover by Studio Pierrot - ANN.
Relevant Details.


  While Viz Media's Weekly Shonen Jump Twitter account had originally stated that the anime would air in 2017, the Twitter account later redacted the statement and said the anime "doesn't have a date yet."


Thus, Black Clover's TV adaptation doesn't have a date yet, but should release in late 2017- early 2018. 

UPDATE: As predicted Black Clover Season 1 is airing this Fall! Enjoy :)