I think the logic is that if something is sustained over time it requires concentration - an earthquake takes some time, whereas a fireball is fire-and-forget.
I'm still a bit on the fence re. concentration. I get the point of it, and more than once I've made a decision to drop one concentration spell and use another one, which I presume is WAI. But the limitation of only one concentration spell is just a bit too fun-spoiling, and I think too many fun spells have become concentration spells.
On the other hand, you'd be opening up a can of worms if the player could learn to hold more than one concentration spell at a time via various means of wizardly growth.
That's why I'm more of a fan of outright removing certain spells from 3.xE games. All the ability boosting spells? Remove them. Other buffs? Don't make them stack.
If a spellcaster wants to pop-off his spellslots, I'm fine if they want to do it. The problem is more with the ease of regaining lost spell slots in most video games (via rest spam), causing the issue of limited spellslots to often be isolated to each fight separately. Once the spell-slot recovery issue is balanced, and further control is warranted over a spell-caster's abilities, tweaking of available spellslots should be the preferable next approach.
But I'll still take concentration over ever again having to cast 'Bull's Strength', 'Bear's Endurance' and 'Cat's Grace' after each rest.