The elements don't need to have everything. If they did, choosing mono would be just a matter of taste and not of strategy.
I'd say, the elements
mustn't have everything. If an element existed which has hard-hitters, CC, PC, Extra draw and healing for a reasonable cost, sooner or later every player would choose this element.
Indeed, every element has a "personality" and this includes weaknesses too. Eg.
can't heal,
creatures have very few HP,
is expensive.
In addition, some cards are essentially stronger or weaker than others but, if we consider which element do they belong to, we might find the reason. Eg. Fog Shield gives 40% miss chance for 2
while Dusk Mantle gives only 10% more for 6
! But,
has Steal too which means you can use your opponent's shield if you wish, also, via vampirization an access to healing...
on the other hand can deal damage damm quick (even without Shards, there is Sky Blitz and diving...) Plus, right now I've talked about only the two elements themselves and haven't examined the possible synergies.