Ego and greed, probably.
Not discounting these writers' duplicity in dealing with their publishers, what's truly troubling when these contretemps raise their ugly little heads is the press's haughty shock and awe that any half-truths (or quarter- or third-truths) should have wormed their way into the sanctity of somebody's memoir. Literary and social critics alike thump their thesauri and behave as if, pre-James Frey coming along and embarrassing Oprah with his million little lies, every memoir published was letter-perfect when it came to factual matters--that no details were added or enhanced (or omitted), that no dialog was fabricated, that nothing was tweaked to make the piece better (or at least readable).
By its selective nature, a memoir is not journalism; it is subject to the tricks our memories play on us; how and why events took place are filtered, consciously or unconsciously, by our prejudices, belief systems, etc. Plus, let's face it, folks: life, by and large, is boring. Even fascinating people have plenty of downtime where nothing of much interest happens. Knowing what to emphasize and what to ignore, where a chapter--let alone the real story--begins and ends (in reality, most people's lives have very few--and very long--chapters), is the writer's job.
And while we're talking about it, the very journalists looking down their collective nose at these memorists are prone to the same refractions they're pillorying; they shouldn't be, but they are. The truth is never more malleable than in the hands of a writer.