Because they gimped their actual game to milk the old consoles one more time, basically. They could have just released on new gen, but didn't. They pretend they re-did all of this, but you can tell, and it's been proven, some of the oldest screenshots use the nextgen and pc assets and graphical effects not available on old consoles (DX9 limitations, dx11 effects in screenshots as far back as then). The reason why so much is "dumbed down" from IV is because when you improve the game but not the hardware for V, but IV already pushed the envelope of the hardware, aspects of the game need to dumb down. Yes, things get done more optimally and that helps... but for the most part, the improvements to V cost a lot of strides IV made and ended up removed so it could milk the old consoles one last time before re-releasing what 5 more times at this point?