Dome is basically the only spell that's rendered totally useless by a superior spell.
Shield costs 10 initially and 5 per second, and it's very useful because you basically get free attacks by reflecting your opponent's projectiles.
Dome costs 15 initially and 10 per second, but all it does is block attacks with no additional benefits.
Shield is also better at close range. As a result, Shield is extremely popular and almost nobody is using Dome. The only advantage Dome has over Shield is that it blocks all around you, but it's not difficult to block the vast majority of projectiles with Shield (unless your opponent is exploiting the hitbox between your shins and you're not using a remote).
The bottom line is that Shield is more useful than Dome, so it would make more sense for Shield to cost 15, 10/s and for Dome to cost 10, 5/s.