Poor sales is the obvious answer to me. Think about it, out of all the music acts you see working, how many use an arranger. Its either a live band, sing to MP3's with a computer, or, God Forbid, a Kareoke DJ !!!!
And for the home hobbiest it would be the price. The BK5 is more than enough for most home players. If they're more serious into home recording a workstation and computer would be a better fit.

Just my $.02
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Bill in SC --- Roland BK9 (2) Roland BK7M, Roland PK5 Pedals, Roland FP90, Roland CM30 (2), JBL Eon Ones (2) JBL 610 Monitor, Behringer Sub, EV mics, Apple iPad (2) Behringer DJ mixer