I think it is due to increased general awareness of arranger keyboards. Advertising is just another indication of the increased awareness otherwise manaufacturers wouldn't be paying to advertise .... like they didn't back in 1999.

I think the other thing that is earning respect for arrangers is the use of sound engines and features from the pro lines of synths. The reputation of sounds on arrangers is now up there with pro synths. I don't think this was so much the case back in 1999, well at least that is how arrangers were perceived back then.