@millie i think because of a combination of secularisation and the preaching work. Churches are running empty in Europe the traditional denominations are dying. A lot of people are dissapointed in their churches, church leaders and teachings.
besides that SDA's do not preach so are not well known in Europe. Mormons do preach but only a very few (only young american missionaries for a two year period). The rest of the Mormons in a country do not go door to door. There are only about 80,000 mormon missionaries worldwide.
Contrary to that all JW's in a country preach and go door to door. So those dissapointed church people are not visited by SDA's or Mormons. Instead of that they are visited by JW's,.. not foreign JW's or missionaries but just local JW's that speak their european languages and are familar with the country and local customs.
That is why there is a difference in Europe between member figures of JW's , Mormons and SDA's