@GaganTopia
Technical explanation:
The Worried Explorer is specifically made for Neksdor ("Town01" grouping, the NPCs you mentioned belong to "Traveler" internally) but he just so happens to be a recurring minor character in the main story (and one of the quest givers in post-game alongside the Youngest Fab Fairy). Mechanically speaking, he's placed on fixed spots on the overworld only relevant to the situation (giving the player the Puzzling Tablet, hints for certain stages/how to get to stages), not for repetitive gameplay mechanic like amiibo scanning, free special grub, pop quiz, (limited) main story boss photo-taking (with repeating dialogue every encounter), and (limited) NPC letter events (also with repeating dialogue per encounter).
So, if you were to ask why the Explorer isn't grouped, it's because he isn't categorized as such in the first place. A better question would be why he doesn't have a unique role group consisting of only himself a la the Dark Lord/Town Guide/Scholarly Pioneer/etc. casting, but that would be weird to see Neksdor Town being the only one of the three towns not counting the Hub to have only 7 residents assuming no replacement. And no, the Genie has his own role group despite sharing the Town01 prefix.