There are several historical factors to consider, particularly the advent of the internet.

We live in the age of information. The gap between rabbinic leadership and the lay is being broached with great rapidity. More people are interested in a more horizontal society wherein they approach sources, analyze them, digest them, etc. and interact on such basis rather than a hierarchic top down, approach. Indeed this very group is indicative of such a trend despite the disclaimer softly heard "but consult your LOR." Put this in tangent with those turned off by the standard x,y,z incarnation of local frumkeit but an inclination towards traditionalism then you've got a real magnification of the "pick and choose" type Judaism which emerged in the 20th c. through the coalescing of many different Jewish communities from various far flung regions in urban centers. Customs, nushaoth, etc. are readily swapped in and out and between members of the Jewish community in a fashion which would have been impossible 100+ years ago.

There is also a great proliferation of groups with all kinds of niche interests across the board. Let's say you are into writing fan-fiction for some indie sci-fi film no one has ever heard of from the early 80's... today you can find others online like yourself with the same interest and develop a group, fomenting the development of a subculture which otherwise would have been relegated to isolated individuals operating as islands of ignorance of one another. Not only that, now there is even the opportunity for others with similar interests with the potential for such intrigue to become affiliated and involved too through greater exposure and the sheer presence of a virtual community. The same could be so for such self-styled "Talmidei HaRaMBaM."

As for why the Rambam as opposed to any other Rishon? Setting aside all of the arguments against the Mishneh Torah that emerged in his own day and in the following centuries, his project was indeed monumental. One thing is that it is the first (and only?) code of halakha to cover the entire range of potential and past praxis based on the rulings of Hazal. On top of which, he does not import local custom into Hazalic legislation nor even Geonic rulings. He may on occasion make reference to them but is always very clear in making such delineations. Such cannot be said of many (perhaps most) other codifications. Indeed it also altogether absent of any Kabbalistic influence - something everyone seems to agree should not infringe upon the autonomy of halakha, yet nevertheless frequently does in other halakhic works of later vintage. Some, such as the students of Hakham Faur in particular take issue with the methods employed by Franco-German Talmudists/Halakhicists and in particular the Baalei Tosafoth (we can read about that here) and admire the strength of purity of the Andalusian mesorah. This mesora is the one that went up in a very direct line through the Babylonian academies to Rabbenu Hananel, to Rabbenu Alfasi, to R. Yosef Megas to R. Maimon to the Rambam. Indeed in the Rambam's introduction to his commentary on the Mishna he claims that he differs from the Rabbenu Alfasi in no more than 10 places and expresses profound gratitude for his and the R. Megas formative impact upon his approach and rulings. In tangent with having this strong mesorah, he was very much so insistent upon following the truth even when in conflict with pervasive and widespread sentiment. Whereas the methods of other Rishonim (especially Ashkenazic ones) would strongly defend tradition as it stands on the ground and attempt to reconcile to Talmudic legislation the Rambam was almost scientific and modern in his approach to texts. Comparing girsaoth, and willing to overturn Geonic precedent when in conflict with Talmudic legislation. Judaism today has become a kind of a shmoregesboard (whether that is lamentable or great a discussion for another time) and many are gravitating towards such a back to the roots (Meqori) attitude that the Rambam's rulings in the MT represents.

Your question whilst primarily concerning halakha ought not discount the appeal that the Rambam's work has for hashqafic reasons. The intellectualist "rationalism" that he has come to represent and shines through even in the MT in the fist volume, Sefer HaMada. Many find therein a clear articulation of beliefs that resonate as fundamentally traditional yet in conflict with many of the ideas that circulate in frum society today. Those who shy away from the pan-Hasidic orientation that has developed (even in the Litvishe yeshiva velt) and the emphasis on ideas with Kabbalistic origins find an ally in the Rambam.

As for why "100% exclusively" the Rambam, I don't think that there are any who really practice as such - rather the MT is being reclaimed as the primary codex of halakha as opposed to the Shulhan Arukh which supposedly all of Orthodoxy "holds by." Rather than relying primarily on R. Yosef's 2/3 method of determining halakha, an approach that a more meqori oriented individual may find fault with, the general launching base and fallback to is to the MT, which is attractive for the aforementioned reasons.