Basically, writers in visual media have a choice: Portray aliens as plot devices, or characters. If you choose the latter, you have to make the character relatable in some way in order to have the audience react well to the story. Particularly if you don't have a long period of time to tell your story.
An example of how not to do things is what Voyager did with Species 8472. When first introduced, they were pure plot device, incomprehensible, terrifying aliens. Then Chakotay eventually found that whole colony of them basically LARPing as humans, and we found out they were completely identical to human beings "on the inside."
Hell, the Borg were another example. Impersonal force of nature the first few times they were featured, but once the Borg Queen concept was introduced, they slowly turned into a generic personified antagonist.
It boils down to the difference between the literary tropes of monsters vs. characters.
A character may be written in a way that's deeply compelling, but the very fact that it is a
character means that in some way they will be familiar and understandable to the audience -- they will act on motives, they will possess personality traits and personalities, they will have feelings, etc. There will be an imagined psychology built into how they are written which will make them comprehensible and possibly even empathetic to some extent or other for the audience.
A monster, on the other hand, is often more thrilling or frightening, but the very power of the monster lies in the nature of the plot device it represents: a monster is danger predicated upon a lack of information and/or upon an inability to comprehend or empathize with them. The very lack of information or of a personality to empathize with activates our feelings of threat, our feelings of "Othering," and our feelings of fear when confronted with forces we cannot understand, relate to, or control. The less we understand about the monster, the scarier the monster is. The more we understand about the monster, the less scary it is.
A really classic example of how these things manifest would be the use of two different types of fantastic dead humans who threaten protagonists in most stories: Vampires and zombies. Vampires may be written to have psychologies that are different from a normal person's, but in the end they are usually characters who act on motives that a human can comprehend, and audiences often build up some empathy for them -- hence all those people out there who love Dracula, or who became fans of Spike on
Buffy. Zombies by definition do not have feelings or personalities nor even much in the way of cognition; there's almost nothing about them that a human can relate to. Insofar as they have motives, that motive is a perverted version of the universal experience of hunger, and their desire to sate that hunger via cannibalism prevents most audiences from feeling empathy for them. Vampires are characters; zombies are monsters.
Zombies are probably scarier than vampires, because we can too easily develop empathy for vampires. And yet, vampires also open up a wealth of creative possibilities that zombies do not by themselves open up. A monster is an inherently limiting plot device; after a while, it becomes impossible for protagonists not to learn more about the monster, and thereby the monster is robbed of the element of mystery that helped produce the fear response, rendering them a plot device devoid of fear. (That's why the real antagonists of long-running programs predicated on zombies, like the Romero series or
The Walking Dead, end up being other humans.)
The Borg are a pretty good example of monsters who were transformed into characters. At first, they were beings whose motives were so foreign that empathy and understanding were virtually impossible; there was no possibility of having a relationship with the Borg or empathy towards them, because there was no real sense of emotion or personality -- like zombies, just an endless desire to consume. The introduction of Locutus undermined this, transforming Picard into a Borg character that we could have a relationship with and have some feelings about, but his temporary nature didn't full transform the Borg; similarly, Hugh in "I, Borg" represented an individual Borg transformed from monster to character, but his transformation did not so transform the rest of the Borg. With the introduction of the Borg Queen, however, the Borg now become, essentially, characters: the drones are no longer monsters in the sense of being threats to the protagonists who do not have relatable motives or personalities, but instead become essentially avatars of one particular character, the Queen, who does have a personality, feelings, and set of motivations that are comprehensible, can be related to, and can even be empathized with to some extent.
