Because Souls series has established a level of success while Horizon was expected to sell around the same as Xenoblade X/Witcher 3. Atleast that was my expectation and it has surpassed both easily.
Nioh is a mixture of Souls based exploration with Ninja Gaiden type combat which makes for an interesting combination. Both of these games are quite popular in Japan so I think the sales are a little lower than what I hoped for it. The game is also choke full of Japanese mythology so there's that =/
Modern Ninja Gaiden has never been popular in Japan though. Nioh has sold about twice the best selling Ninja Gaiden game. Sure it didn't hit Dark Souls heights, but it's still a lot better than most non-Warriors Koei-Tecmo games.
Also Nioh hasn't outsold Witcher 3 thus far. Witcher 3 has had great legs.
30 The Witcher 3: Wild Hunt [All Versions] 67.458
195.039 171.944 23.095 Spike Chunsoft 21/05/2015
From GameDataLibrary
PS3 Ninja Gaiden Σ 40.886 76.095 Tecmo 6/14/2007 (our previous best-selling Ninja Gaiden game)
PS3 Ninja Gaiden Σ 2 [All Versions] 39.327 67.601 Tecmo 10/1/2009
XBX Ninja Gaiden 39.935 56.544 Tecmo 3/11/2004
360 Ninja Gaiden II 27.215 48.945 Tecmo 6/5/2008
PS3 Ninja Gaiden 3 27.796 48.637 Koei Tecmo 3/22/2012
NDS Ninja Gaiden: Dragon Sword 6.100 22.274 Tecmo 3/20/2008
PS3 Ninja Gaiden 3: Razor's Edge 8.743 18.730 Koei Tecmo 4/4/2013
PSV Ninja Gaiden Σ Plus 5.480 18.426 Koei Tecmo 2/23/2012
WIU Ninja Gaiden 3: Razor's Edge 3.012 15.486 Koei Tecmo 12/8/2012
PSV Ninja Gaiden Σ 2 Plus 4.408 9.574 Koei Tecmo 2/28/2013
XBX Ninja Gaiden Black 3.886 7.349 Tecmo 9/29/2005
360 Ninja Gaiden 3 3.793 6.563 Koei Tecmo 3/22/2012
PS3 Yaiba: Ninja Gaiden Z 2.435 2.435 Koei Tecmo 3/27/2014
360 Ninja Gaiden 3: Razor's Edge 1.513 2.408 Koei Tecmo 4/4/2013
Witcher 3: