Here is my view on this and it is slightly controversial: the fans did organise a protest, it was in early 2010 - 30,000 green and gold scarves were sold within like 2 weeks and were brandished at matches along with chants criticising the American ownership and the direction the club was going in. The thing was, at the time Utd. were reigning champions and Sir Alex was still in charge - the protest would therefore have been difficult/confusing for some to back, because if you dig deeper, what had led to the American take-over? In-effect, to protest at the Glazers was to protest against the greatest manager the club has ever known, who continued to bring success even when the under-investment on the pitch had begun, in the last few years of his tenure.
This has continued to the present (although I did read a post in another thread at the weekend, mentioning a certain racehorse & a few years ago that would have probably been redacted). Even after the Moyes debacle, Sir Alex was mainly immune from criticism (even though he was a key factor in that appointment) and whilst he continues to go to the matches and be a key elder statesman at Old Trafford, the fans will not rise up en masse.
Finally - and this is the controversial bit - I wonder how many actual fans who go to Old Trafford on a match day are really aware of the events of over a dozen years ago? Isn't there an issue with a 'prawn sandwich brigade' who are a) not aware of why Utd. are where they are now and b) dare not criticse the 1986-2013 manager and c) are just gutted that the 2 standout teams in the current Premier - are the holders of the Champions league and the current premiership. Furthermore, bizarrely, these teams are rivals across the town and the arch enemy from history (the whole manc/scouse thing) and who will both probably finish 15-20 points ahead of third, let alone Manchester Utd.