There are two main reasons: Number 1, compared to the other princes, he's the most boring. The film was made in 1937, it pioneered film animation, so it's bound to not exactly be chock full of Complicated Characters, that's just the way it is. He's a product of his time.
The other is because ....... *sigh* the annoying online spread of misinformation that he's supposedly 28 or 30 or some BS and she's only 14 🙄
Here's the CANONICAL truth, peeps: NO AGES ARE EVER MENTIONED IN THE MOVIE. EVER. NOT ONCE. If it's not in the text, it's not Canon. We can only assume based off of visual hints.
Now, that said, is Snow White probably one of the youngest princesses? I'd say so, but only because she has that "baby face" and chubbier cheeks. Let's say she's 15-16. Okay. Have you SEEN Princey Boy's face??
LOOK AT THIS DORK. That is NOT a 30 year old creep, that is a BABY! He cannot POSSIBLY be older than 19!! I'd say he's 17-18, max, but he's most likely ALSO 15-16 like Snow!
Snow White and her Prince embody YOUTH and YOUNG LOVE. The spell literally calls for "Love's FIRST Kiss" to awaken Snow White. Not "True Love's Kiss." FIRST KISS. That goes BOTH WAYS, it's THEIR FIRST KISS.
They are precious babies who are each other's first love and I refuse to hear any more slander claiming "isn't he supposed to be 28??" when that was pulled out of someone's ass to make Disney Look Bad >.<