The cost of regular spells don't apply the same to secrets.
Consecrate is 4 mana and Exploding Trap is 2 mana. They do the same exact thing. Except that Exploding Trap is much worse than Consecrate because like all traps it can be played around and you don't use it on the turn you place it.
A trap in Cone of Cold form would be even easier to play around. If you have 3 minions then you would never get more than 2 of your minions frozen.
Putting Mad Scientist into the equation is unfair because now you are adding in another card for combo purposes. It's like saying "man comboing Cone of Cold with Doomsayer means freeze plus INFINITE damage on 3 creatures!" Plus Mad Scientist itself is a terribly overpowered card as it is.
You're not taking into account the value it would have for the class. Freezing Trap is strong enough as it is because it stalls the opponent considerably. Hunter's class itself is about aggression, and anything that delays the opponent's ability to respond is a considerable factor. Hunter's only tools for freezing minions are Frost Elemental and spare parts, from what I know, neither of which are reliable or entirely effective for his class.
Secrets by nature can be played around. That's just how they operate, and it's the intent of the mechanic anyway. It doesn't make them worse than spells, necessarily. You have to weigh the pros and cons. Opponents can work around secrets, yes, but secrets allow you to make moves on your opponent's turn, which largely disrupt their manner of playing their hand and board.
A trap like this for Hunter would be about stall and tempo. Cheap spell (potentially free) to delay a large chunk of your opponent's board. If it's the early game, that will usually mean their entire board.
Not all cards have to be extremely strong. That's not necessarily good design.
EDIT: That said it could probably use something a little more. Deal 2 damage to the minion attacking and freeze it, only freeze the others? IDK. I'm thinking of the current meta with all these 2/1 drops. Seems like an easy way to clear the board and provide stall utility if they're at higher health. Seems like a lot for 2 mana IDK.
