If you're a Fortune 100 CEO, you're a very smart, hard-working, highly successful person. That, of course, doesn't mean that millions of other people aren't also all of those qualities, but if I were running for president, I would want the support of all 100 of them. You (should) always want the smart, successful people on your side.
Well... Many of them, granted, did get there by their own work and ingenuity (though certainly not all of them; family ties will do that for you), but this still ignores that many would want what's best for them. Trump's tax plan definitely fits the bill (he wants to yuuugely slash taxes on the rich after all), and yet these people still refuse to endorse him. Hmmm... I wonder why...?
Because one thing they really don't like is uncertainty and instability? They know pretty much what they're going to get with Clinton and what the effect on the economy is likely to be. With Trump, all bets are off.