Because it is an uncountable noun when meaning:
a strong feeling of interest and enjoyment about something and an eagerness to be involved in it
As the word uncountable says - you can't count these nouns (or rather you can't count the things they represent) you can't say one flour, two flours... The indefinite article a/an always signifies that there is one item. If you can't count them, how do you know that there is only one of them?
When you don't have a dictionary at hand this can help: uncountable nouns don't have plural forms. So, would you say "enthusiasms"? If your answer is no (and it should be) than don't use the indefinite article.