Yaml is very similar to json but json is much more popular and json does not rely on number of whitespaces to define structure of the data. For me, the only nice feature of yaml is that you can add comments (although json developers consider this a mistake, not a feature).
As long as yaml remains a niche format (compared to json) and/or none of the file formats that are important for VTK use it, there is very little incentive to add yaml support to VTK.