Across The Hobbit and the Lord of the Rings, there are three elven rulers of note: Thranduil, Elrond, and Galadriel. The three are all very different in numerous ways, especially because they feature in different books, but there are lots of ways in which the audience can see the same lofty, regal, and experienced characteristics of elves shine through all of them.
But many fans of the films have noticed that Thranduil is referred to as ‘King’ of Mirkwood, whereas Elrond and Galadriel are only referred to as ‘Lord’ of Rivendell and ‘Lady’ of Lothlorien. So why is this?
First, it is important to understand that traditionally, a ‘King’ is the highest position of monarchy or rulership, and that all other titles fall under the authority and jurisdiction of the king. However, it’s not quite that simple in the case of these three elves, for multiple reasons. The first reason is that they are all guardians of their own respective lands. If each of the three members all lived within the same kingdom, it would be very clear that the ‘King’ has the power over the ‘Lord’ and the ‘Lady’, but seeing as they are all in different realms, this makes things a little more complicated. The nature of each place that is ruled over is also a deciding factor that adds complexity into the mix.
Thranduil is King of Mirkwood. Now Mirkwood is a vast and seemingly un-ending forest that is home to thousands of elves, an entire society that has trade with outside lands, protectorship of several creatures in the woodlands, and an internal structure comprising several different functions within the society, from guards to chefs to loaders and unpackers of goods. In this sense, Mirkwood has a large enough capacity and population to be considered a kingdom, and therefore be in need of a ‘King.’
On the other hand, both Rivendell and Lothlorien are drastically smaller in number and in function. Rivendell is known by many as the ‘Last Homely House’, because it was founded as a safe place and a place of refuge after the many wars that the elves fought in. This is also why it is famed for its houses of healing, for it was used to treat the wounded and the struggling after battle.
As such, Rivendell has a tiny population, enough to protect the valley in which it sits, but not enough to be considered a kingdom. Though Lothlorien is probably big enough to be a kingdom, it preferred not to accept status as such for another key reason:
Location. All three elven realms are in precarious positions. Mirkwood is near the Lonely Mountains and the gateways of the ruined kingdoms of the north such as Angmar, where the Necromancer abides. Lothlorien is the closest of the three to Mordor, and also to Fangorn Forest and the once beautiful Isengard, which both become problematic when Sauron begins using his wizarding abilities to manipulate politics and try to claim power.
Rivendell is geographically the safest of the three, but could still be exposed to the north if it found itself too much in the lime-light. Therefore, Elrond and Galadriel both chose to claim lower titles and prevent any possible challenges to the opposition, like Sauron, who may have considered them more of a threat if they were trying to claim kingship, especially considering that both Galadriel and Elrond are of notable decent, as Galadriel is believed to be related to Gil-galad, who defeated Sauron in the Battle of the Last Alliance, and Elrond is a direct descendant of the Valar, and was given the honored choice to remain a mortal elf, or become tied to the fate of humans.
It is also critical to note that there are different types of elves. When Tolkien was creating his world, he created several different types of peoples within the same race. Thranduil is a Sindarian elf, one of the high elves, and as such, has a claim to rule the silvan elves who follow him, whereas both Galadriel and Elrond are a mixture of types, from Telerin to Noldorin to Vanyarin.
This means that Thranduil had a stronger claim to kingship, and that although both Elrond and Galadriel could have claimed this title too, they both decided against it and were happy to be labeled guardians and protectors of their lands, rather than authoritarian rulers. This doesn’t make Thranduil any more powerful than the other two, even though he has a higher title. In fact, Galadriel is probably the most powerful elf in Middle Earth in the Third Age, but she chooses to put that power to the life and vibrancy of her forest, rather than to establish her right to rule.