Because of the difference between "a" and "the". They have a different meaning in English and also in German:
"A bed" is any bed.
"The bed" is a specific bed.
"A" is called the indefinite article.
"The" is called the definite article.
In German the indefinite article is ein- (i.e. ein/eine/einen/etc.) and the definite article is der/die/das/etc. For each, the ending is affected by the gender, plurality and case.
This sentence has the definite article das for the singular neuter noun Bett in accusative case. It translates as "the". If it was going to translate as "a bed", it would have to be ein Bett in this sentence.