1) Yes, this mod brings the ultimate civ experience. I seriously doubt that there will ever be a better civ game in the near future than Civ5 with the Vox Populi mod.
2) The other reason is the one unit per tile combat system which makes war more tactical and fun in contrary to the utterly boring endless stacks of units in one tile in Civ4. Probably the main reason why I never really got into Civ4. The previous civs had at least ranged units (Civ3) or rules that made the whole stack die if the unit on the top lost the battle (Civ1 and Civ2), which was kind of gamey, but at least forced some tactical decision making and need to spread your armies a bit.
However, truth be told, the one unit per tile exposes the flaws in combat AI after you play a little (surprisingly, they kept the same silly AI in Civ6). Luckily we have the glorious Community Patch (part of the linked mod Vox Populi) which teaches the AI to play almost like human.
3) Another reason is subjective, I hated the looks of Civ4. The smug look can be improved by mods, but still once you started to improve the terrain around your cities it looked kind of ugly and very difficult to read, especially when things were moving at least for me, or maybe I am just getting old
I don't like the cartoonish looks of Civ6 too much either. I like the natural and realistic looks of the map of Civ5 the best. Sometimes I like to hover above my empire and enjoy the beauty of it.
4) The hex tiles instead of the square grid. It makes the movement and combat more realistic and the land masses are also taking more natural shapes.
5) Religion: It is much more fun and better thought through in Civ5 (even more so with the Vox Populi mod) than the arbitrary system in Civ4
Been playing since Civ1 and Civ4 has been my least favorite of the franchise. I even prefered the little bugged and imbalanced Civ5 right after the release before the patches and DLCs.