Because "ζάχαρη" is a female noun in Greek, so you must put the female article "η"
http://www.foundalis.com/lan/grknouns.htm You can usually tell by the endings, but some patterns overlap between genders. Nouns ending in -ος in singular nominative (irrespective of stress), for instance, are often masculine, but can actually belong to any gender! ο σταυρός, το τέλος, η οδός. You will simply need to memorize the gender on word by word basis for exceptions like this. On the other hand, if a noun ends in -υ, you can be sure that it is neuter.