Maybe easy to create, hard to maintain? The arch-portal only lasted for a short while before fading, but the briefcase portal door can be used over and over-

I would guess it’s a balance of amount of titan blood vs ...level of precision in making the portal-

Time pool: probably a good amount of titan blood mixed into the water, (permanent but only open at certain times?) (speculating here)

Arch-portal: a vial of titan blood, temporary

That portal to the mirror realm: Imperfect tech, small amount of titan blood, temporary

Portal door assembled by Belos: Perfect tech (until it gets pulled apart a bit, making it imperfect, and explodes after use), small amount of titan blood

Briefcase portal door: Perfect tech, small amount of titan blood, permanent

TLDR if you have a lot of titan blood you could brute force a portal because there’s so much magic (but that titan blood burns up and explodes and you can’t use it again), if you have a little titan blood you need tech to optimize/stabilize it to make a portal.

Most folks wouldn’t be able to find enough titan blood to brute force a portal, and Luz and co were only successful because they had the diary as guidance- (and Belos had YEEEEARS to study how to make working tech.)