From what we know in the movie, Elsa was born with her ice powers. That’s all. No explanation. Not even a subtle reason. Just that she was born as an ice mage.
Originally, there was an explanation for Elsa’s powers. It had to do with planet alignment and it being an occurrence once every 1,000 years. However, it was cut from the final script, giving us the simple “she was born with it” reason we know from the movie.
While it would’ve been nice to have a given reason for Elsa’s powers, I’m not so dissatisfied with the lack of a reason either.
Now, I don’t expect realism in cartoons, let alone Disney, but consider this: how often are people born with certain traits in real life with no other explanation than just being born with it? People are born with a missing limbs or rare genetic illnesses with no reason besides their birth. Some people even consider things such as personality and sexual orientation to be things determined at birth.
Why can’t Elsa have her powers simply because she was born with them? She can. From my point-of-view, ice powers weren’t the only effect. Presumably, her platinum blonde hair came from her ice powers. Granted, that could’ve been inherited from her grandparents, but if she’s supposed to embody winter, it’s plausible.
The movie also implies Elsa isn’t the first in her family’s line to have been born with powers. Assuming a magical family member before Elsa had existed, it’s possible the powers are genetic and can be passed on. Of course, the question then is how would that gene work? I know very little about genetic structure, so I can’t answer that.
Whatever the case, I look at Elsa’s powers, and her struggle with them, as a metaphor of how sometimes, we’re born with things and we have to accept them and embrace them instead of being ashamed of them. There is no point in the movie where Elsa wishes she didn’t have her powers. She constantly worries about harming others with them, but she never actively wants them gone, and Let It Go is essentially one big, happy musical number for her as she’s embracing her powers for the first time in over a decade.
If the planetary alignment explanation is included in Frozen 2, it’ll probably please a lot of fans. Personally, I’m neutral toward it because even an explanation brings up a dozen questions.