Previously, one had to use a slow/expensive wire transfer to Ireland. The fastest way of funding was, in fact, to literally mail a check, and all kinds of annoying paperwork was required.
Now even that isn't good enough, because the company offering the service closed.
However, while InTrade existed, it did experience healthy growth over time. So I think that, unlike most other tech projects, interest isn't actually the bottleneck.
Although PMs do sometimes have a problem with low interest, I believe this is more "conflict of interest" than "lack of interest" and I am working something which explains that in more detail.