Exactly, is just easier for Yamaha to choose a Vtuber because they already have all needed data (number of subscribers, public comments, active users supporting them..) for starting a crowdfunding in a "safe" way.
For example, if a Vtuber has 100k subscribers, Yamaha probably thinks that 1k or 2k of them might be happy to support the crowdfunding. Some UTAU characters are surely popular, but is more difficult to understand how many dedicated fans are there.