That's because the spells have a limit, after which an increase of intelligence will no longer increase the spell's power. When you see, "You have maximized this spell's power", then you can no longer get any more Dmg out of it with any more intelligence and must move to the next level spell. You may even possibly have times where a lvl 1 is more powerful than a lvl 2, but after a few levels and more intelligence the lvl 2 will be better.
Just remember that increasing intelligence increases the Dmg, but there is a limit at which intelligence no longer adds any more Dmg, that's when you need to get the higher lvl spell.
And since you are not playing a pure mage your intelligence isn't maxed when you get the next level spell, that's why you are noticing it. If you had a pure mage with max intelligence then you'd probably max out your spell before you could find the next level version and would then be here asking how come you can't find a level 2 spell yet because your lvl 1 is too weak.
