Films and videos are a totally different area than concerts.
And his worst tour. Why to put him at his worst in the cinemas when he had lot better tours? It would just unnecessarily open him up to bashing and criticism.
It does matter that the Wembley'88 show was of poor quality. At this age when people buy Blue Rays it does put people off if a show is released in VHS quality. But that doesn't mean they will cheer it on if another show is put out in excellent quality but the show itself is poor. Then they will criticize the performance.
Also I think MJ fans sometimes have irrealistic expectations as of how many copies something should sell. Old concert DVDs are not necessarily huge sellers, they are usually targeted to the fandom of an artist. Queen's Hungarian Rhapsody is excellent quality, recorded on film, so it could also be released to the cinemas and it's a great show too and still apparently it wasn't a huge seller. At least when I compare the chart performances of Hungarian Rhapsody and Wembley'88, it seems to me Wembley'88 actually did better despite of the poor VHS quality:
http://en.wikipedia.org/wiki/Live_at_Wembley_July_16,_1988#Charts
http://en.wikipedia.org/wiki/Hungarian_Rhapsody:_Queen_Live_in_Budapest_’86#Charts
Now, imagine how well it would do if the Estate could find a great show in great quality and release that to the cinemas and promote it properly. But the show has to be great and represent MJ at his best and not at his worst.