It's because artwork is like clothing - it's the first thing that player sees and it's the first thing that will make a potential player to invest their time into the game, to give it a chance. Just like in real life, the handsomer the person that you first time see, the easier it's for you to give them a shot, and only then do you judge their speech (dialogue) and behaviour(gameplay).
And just like in real life you will choose to stay with the person that is interesting to you not only visual, but also that fits you morally and can sweet talk you. Gameplay can be hit or miss though, there are succesfull games that have great story+dialogue+art, yet have horrible outdated gameplay. Same for dialogue, hence projects like Rack2, BotN and Fallen Doll that have almost to no context, btu their other features raise their projects higher to a success.
If artwork is bad though, then less people will even try it out to begin with, thus they won't even experience other features that game might provide. And that might be the reason №1why game might fail.
Unfortunately the very first impression is very important which is why it makes sense to concentrate on the graphics at first and only later start adding other features.