I merely sketch a partial answer to your question:
There are quite a number of distinct reasons, but three important political/bureaucratic ones are
(1) There is no simple bio-marker for the disease (as there is for diabetes, for example)
(2) It saves a lot of money not to help several millions of ill people
(3) "We have all sufficient strength to support the misfortunes of others" (Rochefoucauld)
What is disquieting about it, in a so called civilisation, is that - in spite of all the evidence on the internet, which makes things radically different from 32 years ago when my former wife and I both got ME and had to wait for 10 years to even know of its existence, in spite of having outlined our symptoms very precisely to tens of doctors of medicine - the following gambit is still being played, as it has been for decades now:
Either you are a nut-case and we will force you to work, exercise and psychotherapy, or you are not, which is tough luck for you and not our responsibility anyway, so tada!
It is true that in Holland in the late eighties, where the wellfare systems at the time were very good and much abused (more by employers than by employed by the way), I have met some people with ME I thought to be malingerers, but since then the wellfare systems have been made much worse here.
In any case: I have NEVER read or heard of ANY rational discussion of the issue: OK... there are malingerers, there are liars, there are people with unexplained diseases - so how do we sort these out rationally and ethically, that is: without victimizing the genuinely ill?
The normal answer in Holland is: "We have them seen by an objective medical doctor, not their own GP or specialist, and they decide". That seems fair enough, in a way, until you realize that:
(a) These "objective medical doctors" are usually from the medical dregs i.e. the worst doctors who are unable to find better employment
(b) These "objective medical doctors" are forced to declare a pre-determined percentage of people who say they are too ill to work as fit to work (pre-determined politically or bureaucratically)
(c) There is no realistic appeal against their decisions (court are overworked anyway; judges are not medical people; going to court requires money and health)
(d) These "objective medical doctors" are these days in Holland employed by private firms and private insurance companies whose interests they serve (and their interest is to pay out as little to ill people as possible)
(e) Most things related to medicine in Holland over the past 10 years have been massively turned over from the state to the health market, on the basis of the argument that "folks are then free to choose the best from genuinely competing firms" (as if selling cars and helping people medically are on a moral and economical par)
(f) There are now in Holland, and apparently also in England, and quite possibly elsewhere, lots of market forces in the form of commercial healthcare-firms who are paid from the taxes to do the bidding of the state.
For example: Next year, when I'll be 61 and won't have worked since 1978, four years before becoming a state pensioner, I again must prove to some medical idiot that I am unfit to work, on the basis of the argument that by not working I have proved I am "not integrated in Dutch society" (that has refused to help me in any way whatsoever, in spite of my being a medical invalid all these years according to medical doctors who know me and specialized on ME/CFS, except for minimum dole).
No employer will want to employ me, given my age and "career" and a considerable part of the healthy population unemployed because of the economical crisis; if what I say and have said or 32 years now, and what my doctors agree to is true, I can't even GO to work by public transport, not to speak of WORKING; and in fact forcing me to work = forcing me to commit suicide.
But even so I must again be seen by some medical idiot who must declare 80% of those he sees as fit to work, and if I protest that merely shows that I want to be difficult, am impolite, and a nut-case, but even so therefore not unable to shuffle coal under strict guidance of strict overseers, since physically, they will say, nothing is wrong with me, medical omni-science has abundantly proved, and besides I OUGHT to want to be "integrated into society", for every decent Dutchman wants that.
The underlying reason for this cruel idiocy is that there are now a number of commercial firms in Holland, often started by former civil servants, out for millions for themselves, who are paid 10.000 euros per head for "reintegrating people into Dutch society", by hook or by crook, under their own commercial steam, without control (and who dares to complain therefore and thereby proves he or she is fit to work: Catch 22 - you are a malingerer until it is proven that you are not, which as things are now in Holland, requires your death, or at least the loss of four limbs, and even then you ought to be "integrated into society" and "contribute to it", for if you are not you contributing by working in this Dutch Paradise, you are evidently an asocial pathological case who, in his or her own interest, ought to learn Dutch "Norms and Values", our brilliant and beautiful Balkenende PM proudly claims, by being forced to work).
In short... the brief of it is that one is stigmatized, maltreated, not listened to, denied one's human rights, denigrated, and treated as a malingerer who refuses to be "integrated in society" BECAUSE the commercial firms make money that way for the state, and the state saves money that way "to keep our Western civilisation afloat in these times of crisis", the state's spindoctors will claim.
It's homo homini lupus, but with a nice smile, a moral patter, a medical coat, and a script written by the likes of Peter Mandelson or Simon Wessely.
I hope this explains a little, and I hope the Canadians are not as crazy and cruel as the Dutch, and indeed the Canada Criterions are fair and sensible in principle.
P.S. Background info (for those - mostly - unfamiliar with M.E.):
Title: M.E. - Documentation