Few comments:

Excellent topic.

1- In the US musician tend to group together in Bands. It can seen all over the place. One of the atraccions of bars and restaurants are "Live Bands". We don't see advertising to "JoeDo playing live".
2- Price. If we look prices for high end arranger and high end WS, the arrangers are always more expensive. Korg M3 ($2200.00), Korg PA800 (2800.00), Korg PA2x (3400.00). Why then if I am going to play in a band I will purchase an arranger? If we look in the "used" market the difference is even bigger.
3- I think that Churches are an important sector in the music market. In the area where I live there are probably 500 churches. Imagine that amount of churches performing music every Sunday. In 99 % of then the music is play in groups (bands, etc) and 99 % of then use WS.
4- Peer pressure. Because they don't know, usually musician tend to look some body playing an Arranger as a weird guy, some body that does no have the skills. Some time they call you instrument as " you Casio type keyboard". Even here in SZ, I have read reviews of arrangers that look "more professional" because X model have no speakers and the "no professional" have then.
5- As Diki say, poor marketing in the stores. Many times, when visiting music stores, I have to volunteer, to show a interested customer what the Arrangers can do, because the sales people don't have a clue.