There are several reasons why alpacas are popular and just one reason why they might not be so popular. I’ll get the unpopular bit out of the way first: they spit.
Alpacas are known to have the habit of spitting which humans consider to be unpleasant. They basically spit vomit which IS unpleasant!

A farmer who lives with alpacas tells me that they do this under two situations. Firstly, when one alpaca interferes with another alpaca’s feeding and tries to take their food, they will give the interfering alpaca a warning that a spit will come unless they back off. If they don’t back off a spit arrives in their direction.
The second reason is that when a female alpaca is pregnant, she tells the male that his advances are unwanted by delivering a large wad of spit at him. It’s a form of alpaca-to-alpaca communication. Nice 💔😢.
If a person is a recipient of alpaca spit it is an accident. Perhaps the person is in between two alpacas who are communicating with each other about feeding and they are an innocent bystander and victim of alpaca communication.
Those are the negatives as I see it. There’s a long list of positives one of which is to do with functionality. Firstly, the look very cute! A major factor in their popularity.
Alpacas are well known for their rich and fine fibre. They have a beautiful fleece which apparently is very smooth and versatile. It is more resilient than mohair from the Angora goat. And it is more refined than Kashmir and shinier than silk according to a great advocate of alpacas.
And the alpaca character is attractive. They are said to have gentle, calm and affectionate characters. They make nice pets.
Although, they don’t particularly like being petted. I guess this is because they are not like domestic cats and dogs being fully socialised to humans.
Although the alpaca is domesticated, they are not fully domesticated vis-à-vis the animal-human relationship.
But alpacas have a serene manner. It is said that they are torn between curiosity and shyness and often curiosity wins out.
There is another feature about the alpaca which comes across quite strongly: they smile all the time!

This is due to the anatomy of the face in my view. Although they may be happy as well! But truly, this is a lot more about their facial anatomy. There may be an evolutionary factor in play which is that they are projecting an amiable disposition to other alpacas which means that they are friendly which in turn helps to create a harmonious alpaca society!
Do alpacas like humans? This is about socialisation. Animals instinctively are cautious around humans until they understand that they are safe. Animals should be cautious around people because human is the world’s most successful predator and en masse they are incredibly destructive of nature.
So, alpacas are prepared to like humans, but humans have to show that they deserve to be liked.
Below are some more pages on the genial alpaca.