The reason being that any additional bomb cards are only 5 damage each. You're out of doubling mechanics with just the two Clockwork Goblins. The Bombs are only here to be able to grind out wins against stuff like Mecha'thun and Jade Druid or anything else that either draws the entire deck or that does the complete opposite and doesn't fatigue (or at least fatigues way later). If 40 damage worth of Bombs isn't enough to outright win at some point you should be focusing on the board.
See HearthPwn for the full discussion / revision history of the deck.
The previous version of this deck did include one copy of Seaforium Bomber. I ended up taking it out for Nexus-Champion Saraad.
The reason being that any additional bomb cards are only 5 damage each. You're out of doubling mechanics with just the two Clockwork Goblins. The Bombs are only here to be able to grind out wins against stuff like Mecha'thun and Jade Druid or anything else that either draws the entire deck or that does the complete opposite and doesn't fatigue (or at least fatigues way later). If 40 damage worth of Bombs isn't enough to outright win at some point you should be focusing on the board.
See HearthPwn for the full discussion / revision history of the deck.