I also don't understand why market cap or block rewards have anything to do with this. A 51% attack on the network doesn't mean the attacker would make more BTC than they could otherwise "legally." An attacker would only achieve one of two things:
1. Double Spend
2. Damage to the Ecosystem
In both cases, the incentive remains the same regardless of the price of Bitcoin or the block rewards.
I understand that an attacker wouldn't gain any bitcoin from attacking the network. But bitcoin having a larger market cap should make it a larger target. one persons lose can be another persons gain. Maybe the relationship is not 1 to 1 but market cap and security should scale together.