At some point, the reviewers switched to the Newcastle-Ottawa Scale for rating the literature included in their reviews. In the reviews, there is no reason given for this that I can see, which is certainly not best practice. There’s a belief going around online that the reason that the reviewers switched the scales is because the MMAT recommends against excluding low quality work, while the NOS has no such recommendation.
As someone who does systematic reviews professionally, this argument makes no sense to me. All rating for bias is to some extent subjective. While both the MMAT and NOS attempt to create some measure of objective ranking for research, they are both ultimately up to the judgement of the reviewers who are using the tools. Changing your rating scale isn’t going to magically change the conclusions of a systematic review, especially when the review uses a narrative (i.e. subjective) synthesis method anyway.
In addition, as I noted above, including low quality studies in these reviews probably wouldn’t change much, because the low quality of the papers reduces their usefulness anyway. I very much doubt that anyone cared enough about the rating scale to switch it for nefarious reasons - the most likely explanation is that they didn’t find many qualitative studies in their searches.