simple

Wikipedia has a lot more donors, but also their costs are probably cheaper than Signal. They mostly host text and decently compressed images, Signal uses way more bandwidth and people share high quality videos and other huge files. The servers are very expensive, there were articles estimating their cost to be around $50M per year.

Makes sense., something as huge and expensive as Signal can't run entirely on donations.

Is it out yet? Last I checked it was a demo only.