...Because the last entry in the series came out almost 20 years ago and the brand has had no cultural presence since then?
I'd like to see it myself, but I don't see how a few samurai-era game successes would prove bank for this brand. Capcom has done a good job adapting old RE overhead location designs for third-person combat/exploration mechanics, but Onimusha (and Dino Crisis, for that matter) would have some challenges RE didn't deal with in the perspective shift and the increase of tactical awareness / aim accuracy. (Dino Crisis at least has the iconic Regina and also kids buy dinosaur products, so that one's weird to not remaster even if it would change the game a lot to leave the locked camera system.) Onimusha 4DoD did a 3D world / behind-the-back camera but it was clumsy (and a remaster/remake would still need that tight-to-character camera design if it went third-person since the combat zones are so small and limited,) and it's unlikely an old-school top-down camera would be used in a AAA game today.
This makes remakes a difficult option (albeit not impossible,) leaving choices of continuing the series with Onimusha 5 or rebooting the franchise. Neither of these sounds like a billion-selling proposition unless there's a compelling new idea to build upon. Just banking on a small run in samurai titles from other publishers (many of which are very different games, aside from the setting, ) doesn't lead Onimusha directly to the top purely on nostalgia.