Bequests/legacies are a key target for building long term stability in a charity - as distinct from year to year survival from one off donations or subscriptions. Whether there are particular disadvantages for ME/CFS to attract bequests I'm not sure, again this come down to what the comparators are. If the comparators are the big killers cancer and heart disease then ME/CFS loses out simply on the basis of numbers - in the developed world lifetime prevalence of cancer is 50%, while cardiovascular disease is the greatest single cause of mortality; very simply there are a lot of people either affected (patients and relatives) or who are anxious about diseases, amongst whom a proportion are prompted to make provisions in a Will.
It may be true that PwME have few resources to bequest, however bequests to medical charities are as likely to come from relatives and freinds as from patients. A case might be made that the isolating effects of ME/CFS means that social influences which prompt bequest making for other illnesses are absent and that by the very nature of ME/CFS, PwME are not energetically making the case for fundraising, especially absent being the heroic survivors such as those of radiotherapy or heart surgery.
I still see the underlying problem being the invisibility of a large proportion of the estimated patient population. One might say (nod to
@DigitalDrifter,
@Jaybee00) that estimates vastly inflate the numbers and there are simply not that many of us - if that is the case, at least in the UK we are in fund raising terms punching above our weight. However I don't think this is the full story, again using the UK because we have data, between them the MEA and AfME have maintained a long term member/supporter base of 8k to 10k, these are PwME plus friends family etc.
Figures (applogies can't find the link) from I think 2017 gave reason for UK sicknes benefit claims - CFS as 50,000, that number would exclude patients with higher earning spouses, under 18s and those past retirement age. The implied total may be significantly lower than the often quoted 125-250k ME/CFS patients for the UK but it still suggests perhaps only a tenth of PwME in the UK are involved with patient orgs and that the vast majority of the freinds and relatives of PwME in the UK have no formal relationship with patient organisations.
One explanation of this low level of engagement may be a disproportionately high level of ME/CFS prevalence within social groups that culturally have lower levels of engagment with formal structures such as non minority framed Health Charities. Some of the (US) epidemilogy data suggests this disproportion is a feature but the image of ME/CFS as illness solely of articulate, socially engaged people persists.