Running for president is very expensive. All other candidates are also spending a lot of money -- but they're spending a lot of _other people's money_, to whom they are now beholden. What do those other people expect to get in return for their financial support? Surely they don't just give away millions of hard earned dollars to install someone in the most powerful office in the world out of faith in democracy. Donors and their groups -- especially the large and important ones -- are almost always aligned with explicit policy (and often persoanl) quid pro quo expectations, should the candidate win the election.
Bloomberg, by contrast, is spending his own money. He's not beholden to anyone's interests or greed or kickbacks or special interests. Whether you like or dislike his policy proposals, surely this point, at least, is better than a candidate with a wealthy donor network of obscure motives pulling the strings behind the scenes.