Because it's 2018 and media format support in smart TVs is still garbage. I have relatively new Samsung and Sony TVs, and they are both really finicky about what they support - various combinations of container format, video codec, audio codec and bitrates work on only one or the other, and plenty works on neither (including 10-bit video).
Same thing with a media PC.
You could get some transcoder on your NAS (Plex?). Newer TVs support newer formats. Last year's LG OLED supports everything I could throw at it.