I'm learning Hebrew, ok, but I think it's because it's a construct phrase.. and the hey is needed because you put it on the second word (in a construct phrase). For "the". (And/or) Because you need a hey because of the possessive pronoun, you always add a hey to that word unless it's a name or family member.
I'm sure an Israeli will be able to clarify. It's probably in the tips and notes or in the discussion board. Update: it's in the tips and notes, but I don't think it's explained particularly well there. Just, the hey is needed to be "definite", and in a construct phrase (or construct state) it goes on the second word.
Here are some explanations (I do not support or advocate for the sources or sites. They do seem like good articles. My disclaimer):