Because the highest cost a card has is occupying a space in your deck. The more valuable it is, the more bang for your buck do you get.
Additionally, that second deck is low on quanta. It'll pump out creatures and Poison slower than the first and they'll be weaker to boot.
Elements is kind of special in that every element has access to high attack creatures (Dragons). Because of this they can all rush to some degree. However, as you can see it's mainly thanks to Mummy that Monodeath is fast (and Recluse when upped). Middle-ranged attackers are always the main focus of rush decks in all card games because they come out early and do decent damage. This translates into more total damage in a short game than a more expensive high attacker or an even cheaper low attacker.
However, your examples are bad. Something with 2 attack would be better for comparison with a mid-range attacker, so let's take Physalia as an example. Compare it with Toadfish. They may actually be equally fast if you have towers because quanta is instant and Physalia only costs 1
, but how fast they are depends on how long the game is. The shorter it is, the faster low and mid-range attackers are. Because around 100 HP is the most common, mid-range attackers are usually the fastest.
