Subjective, just like your's and mind. Each one of us have different things that are important to them and we all have our biases, our likes and dislikes. There's two things that could be at work here and this is subjective on my part. One, as you and Gallup pointed out, Bush has been out of office for 8 years and has remained silent. For a lot of folks, Bush is ancient history. At the end of his term he was remembered for the recession and hence very unpopular, but at the beginning or in his first term he oversaw 9-11 and a united America. Memories are those of which each one of us wants to retain. I know myself, I tend to remember or keep the good, forget most of the bad as time goes by. I think being out of the public eye has been the biggest help to Bush's rise in his favorability among Americans as a whole.
Two, who do we have to compare these last two presidents with? Donald Trump. Now as the article pointed out, when Bush left office, Republicans still had a very favorable view of him, Democrats and independents did not. Democrats have a negative view of a president of the other party is to be expected. Independents in my opinion usually reflect the bell weather of America, not those who are Republicans and Democrats when it comes to these things. Independents gave Bush a 29% favorable rating when he left office, a 56% today. Perhaps a case of absents makes the heart grow fonder.
My most important issue has always been the national debt. Bush added approximately 6 trillion, Obama 8 trillion, both president's get a big fat F from me. Other things rose both presidents from failures to below average as far as I'm concerned. I would only place Carter and Ford below them in presidents I have experienced in my life time. I was born right after WWII.
