Hera is not the only goddess of her generation, being a sister to Demeter and Hestia. Furthermore, Aphrodite, in the Hesiodic account, is older, originating from the batter of Ouranos' severed junk when it was discarded and cast into the sea.
Why is then Hera queen of the gods?
Without touching on the issues attendant to the idea of someone asking a question with 'the answer' already in mind, let alone trying to suss whatever that might be out, I'm going to answer as cogently and enjoyably as I can, even if the range of potentials I give might make it uncomfortably close to opining.
I would preface this by saying I'm answering for Hera in the Greek mythos, not Juno over on the Italian peninsula, though I suppose a lot of the same reasoning might apply.
To begin with, and buttressing yannis's comment, there is an appreciable amount of merit in the idea that Hera is queen of the gods because she's the consort of Zeus, who is the king. This is very linear, though, and comes out sounding like premature certainty (hey, it happens to every guy a few times in his life).
Moving along, considering Hera's attributes on their own, there is some reason to call her the queen. The first argument I'd advance is based on her 'birth order'. Greek tradition varies on this, but one of the most popular is that Kronos and Rhea had six children, first three females and then three males. In this tradition the order is Hestia, Demeter, Hera, followed by Hades, Poseidon and Zeus. Hesiod's Theogeny favors this order. However, Kronos's bad child-eating habits, and Zeus's freeing his siblings from their father's stomach later on, means they have two 'birth' orders - the first reckoning from Rhea's vagina, the second from Kronos's mouth - and in this sense Zeus is both the youngest and oldest. This is, incidentally, one of the bases for Hestia's epithet 'First and Last', but more to the point places Hera as the first, and therefore primary, of the goddesses.
Moreover, we ought consider the somewhat murky etymology of her name. If it derives ultimately from a feminine form of 'heros' - as in 'the ascendant male' - then she is the ascendant, and paramount, female.
Lastly, and maybe most importantly, we must concern ourselves with Hera's bailiwick as goddess of marriage, domesticity, womanliness and, in an attenuated sense, motherhood. If a king is the 'father' of a nation (or if he is popularly seen this way) then a queen would be a mother to a nation. By the reflexive property, a mother would be a queen. Most mothers' 'royal' prerogative extends only so far as their property line or the influence over their relatives and social circles, whereas Hera's would encompass Olympus and the 'nation' of the deities who rule from there. This cannot be seen to emanate from Zeus's authority, as each is necessary for the existence and validity of the other, and the fact that Zeus commonly carries the epithet '(husband) of Hera' stands as evidence that Hera's power/prerogative/authority is distinct from Zeus's, even if the two end up irrevocably intertwined.
Cheers!
While some of the answers here are on point, I would like to add a bit of complexity. According to Homeric poems, Hera is not considered the queen of gods and mortals, this association appeared in later myths. See theoi article on Hera:
...after the marriage with Zeus, she was treated by the Olympian gods with the same reverence as her husband. (Il. xv. 85, &c.; comp. i. 532, &c., iv. 60, &c.) Zeus himself, according to Homer, listened to her counsels, and communicated his secrets to her rather than to other gods (xvi. 458, i. 547). Hera also thinks herself justified in censuring Zeus when he consults others without her knowing it (i. 540, &c.); but she is, notwithstanding, far inferior to him in power; she must obey him unconditionally, and, like the other gods, she is chastised by him when she has offended him (iv. 56, viii. 427, 463). Hera therefore is not, like Zeus, the queen of gods and men, but simply the wife of the supreme god. The idea of her being the queen of heaven, with regal wealth and power, is of a much later date. (Hygin. Fab. 92; Ov. Fast. vi. 27, Heroid. xvi. 81; Eustath. ad Hom. p. 81.)
Emphasis mine
Aphrodite seems like a goddess that was added afterwards in Greek Mythology. It seems that she was added to represent eastern goddesses like Astarte and Inanna-Isthar.
Hera was not the first wife of Zeus. Metis was. Later on Hera took the place because it was the only way she would make love with Zeus. It was that simple.
Goddess of love can not become the mother figure, the perfect wife figure of stability caress and fierce wrath to protect her family. It was Hera for a good reason. Hera was feared, Hera was respected. So she was symbolizing the holiness of marriage.
Aphrodite could not play that role.
