Let me ask this another way @Concerned ... Why is Dubya being rehabilitated ... and by who?
According to the poll data I provided, 43's favorability has remained about the same with Republicans but it has increased with Democrats and independents. Personally, I've seen no evidence that anyone is "rehabilitating" Bush; however, as the article I linked noted:
It's not uncommon for a president's favorability numbers to improve after leaving office.
To paraphrase the old saying: time heals many wounds. I think that's all we're seeing with Bush just as we've seen it with many other former Presidents after leaving office.