2 miner (discus fish and ghash.io) controls 40%-60% of the Bitcoin network, how is it easier to collude with 51 delegates?
Delegates are usually highly reputable members of the community, they also earn a small amount of fees for being a delegate, and at the first sign of dishonesty, they will be voted out. If they can't be trusted, why would you ever trust an anonymous miner who possibly have 0 stake in the eco-system?
Those aren't miners but mining pools. Mining pools have historically grown and faded over time and miners can switch pools and often do instantly.
The beauty with bitcoin is I don't need to directly trust the intentions of people or delegates. Trust is built upon the fact that there are great incentives to play honest and it costs real money to make an attack. With DPOS or POS their is only the first and not the second, thus Bitcoins weakness is also its strength as an added layer of security.
Whether or not one person can be trusted as a respectful member of the community or not should should not be the concern. We should focus on decentralizing bitcoin to a point where trusting a small group of humans isn't needed as all.
Delegates can have the best of intentions and be corrupted over time, by special interests(lobbyists), misguided decisions, changes in character, or simply being a target by a hacker who holds them hostage or gains access to there vote through compromising their systems.