Who knows for sure? There were several reasons imo, as follows:
Bitcoin scaling issues are finally catching up to it. It can only handle about 1,000 to 1,500 transactions every 6 minutes. So they know that is an issue and they are trying to fix that with the side chain called Lightning. In the past, that may have not being as big an issue but as other blockchains now come online that can do better and faster, many that use bitcoins are indeed switching to those alternatives.
Bitcoin transactions are getting more expensive. You are finding people paying up to $20 and more for transactions just so they can get mined. Part of the attractiveness was its speed and value to tansfer funds. What would be expected if it begins to get more expensive than WU?
Some of the regulatory action such as the Chinese ban on exchanges and some of the FUD might have began to affect actual adoption.
We are still observing and wait to see if this level of adoption growth rate is a temporary issue or a long term realignment.