Blake Zeff outlines why candidates go off the script for big donors:

[D]onors are given outrageous — though, usually, completely superficial — promises, in return for their largesse. For example, a “high dollar” campaign fundraising event will be billed as a chance to hear the “real inside scoop” of what’s going on in the campaign. So, candidates are often admonished by their fundraising staff not to give their usual stump speech, because these donors expect more. It’s the same reason big money folks are invited to take part in “strategy” calls with campaign staff, who run them through the latest polling (most of it publicly available) and offer optimistic assessments and broad strokes about the strategy moving forward. Which means that when a candidate attends a fundraising event hosted by a top donor, he or she usually deviates from the stump speech, talks politics, speaks casually, and tries to give the attendees the feeling they’re in on some insider campaign scoops, as an enticement to get invested (literally and figuratively).