"it isn't popularly used"? It is the Top 10 used Pokemon, more than even Lugia, so it's unfair to tell that it's not used.
Thanks to Heavy-Duty Boots, Ho-Oh has established itself as a staple of the AG metagame. An immunity to Stealth Rock granted by this item allows it to make full use of its fantastic ability, Regenerator, to switch into a large portion of the metagame. Ho-Oh is very effective at assisting its teammates when coupled with its solid array of support moves. Sacred Fire is an excellent STAB move due to having a high chance of inflicting a burn. Along with Toxic, this prevents a large part of the metagame from using Ho-Oh's presence as an opportunity to switch in.
Due to how popular are Yveltal and Calyrex-S in the meta, Lugia's role of Defogging will diminish, because of its disadvantage to both, it can't do anything to Yveltal except giving a paralysis or Toxic poison, and Calyrex-S can simply use Substitute. On the other hand, Ho-Oh can pivot around both, since a spread of 252 HP/52 Sp.Def takes like 40% from Max Darkness. Overall, it can be, one of the best Defogger in the format.
However, the lack of popularity in Nat Dex AG compared to Gen 8 AG is due to Primal Groudon, Kyogre. Primal Groudon always likes to carry a rock-type move, and Primal Kyogre basically easily beats it due to people running Sacred Fire over Brave Bird.
Hope this helps!
