I'm not saying that a piece of art that no one ever cares about after it's created is valuable, but it's hard to judge the value of art based on current popularity. Plus, the judgement of popularity is influenced by the culture at a given time, and the existence of a given work or works might have a pretty big impact on the course of that culture's evolution -- regardless of its initial ability to gain funding.
There are also the practical difficulties of making sure that the funding request gets in front of the people who'd appreciate the work in question, and that they have the money to be able to afford to donate, and that they're not disinclined from donating for reasons that have nothing to do with the artwork in question (e.g. they think kickstarter is some phishing site, or think there's no point since they can only give a very little bit).