I've often wondered if they'd got in trouble over the tricked into suicide angle and were forced to bring Ace back, but as no one ever mentions that I guess not.
I was under the impression that Wagner and Grant had grown to really dislike Ace (there's a certain point - maybe Strike! - at which his character notably changes and he becomes a lot more weaselly, signalling, I suspect, their contempt for him) but readers absolutely and quite rightly loved the series, and besieged Tharg for its return, much as depicted in 'Whatever Happened to Ace Garp?' Whilst 'The Doppelgarp' is largely poor, it has a few strong moments, particularly in the opening episodes: the death of Ace's scarf is oddly touching. 'The Garpetbaggers', however, is complete mince - I love Ace Garp, but I'm not sure I've ever managed to sit down and read that story from start to finish in one sitting. However, I'd argue that the whole 'parallel universe' stuff is redeemed by the final story from the '89 annual, which is absolutely beautiful in terms of Massimo's full-colour art, and which brings the series to a perfect conclusion. (Wagner and Grant clearly still hate Ace in it though!)