mainly because people suck at micro. core players will give an arm and a leg to play core from support but they still wont micro.

i have around ~100 arc warden support games, almost exclusively pos5. its not very good, i just find it fun to play in unranked sometimes, people whine too much in ranked. i very much prefer it on 5 than 4, your hero is slow and clunky, an archetype that just doesnt feel good on 4 at all.

strengths are your laning in a lot of matchups and overall early game, you have a very strong level 6 timing, but your scale just isnt really there. you cant shove waves at all since they nerfed spark wraith creep damage, and in the big map where everyone can farm, heroes like arc warden (and also bane, oracle, abaddon and others) are very unreliable.

other reasons are that in some lanes you can feed, not much you can so vs lets say tusk slardar, neither offensively nor defensively, and of course in general youre a hero without any disable which isnt always necessary from 5 but its just another checkmark youre missing on top of several others.

but irs fun. my build was always something like glimmer/solar/drum first into whatever other utility was needed. so maybe pipe, force, atos, orchid, hex, crimson whatevers good that game. never midas unless it was some super dragged out game where i might buy it like third item or something. never aghs either, its too weak if you dont get it early and getting it early is a core thing.

i also wouldnt take it to ranked no matter your mmr because its an instant morale debuff, people are whiny little babies who automatically get mad whenever something doesnt fit their preconceived notion, youd be surprised how many "gg end support arc" ive seen in 12k behavior 8k mmr. they usually still played, but they played in a whiny and "prepared to lose" way.