I think a lot of pro keyboardists in bands want to take much influence on the sounds they play, really be able to edit them in depth and sometimes create completely new sounds in case they are into synthetic, electronic sounds.
Arrangers simply do not provide the editing architecture to do that. Even a Tyros doesn't provide the architecture to go down to the level of individual waveforms and combine them freely to form voices, unless you use the voice editor via computer.
In a MoXF, I can take up to 8 waveforms, let's say trumpet, alto sax, mute trumpet, trombone etc. to form a brass section as a voice - and then I can combine 16 voices to form a performance (or mixing, or studio set, or combination in the terms of Korg). So a performance/mixing/combination can theoretically contain 16 x 8 = 128 elements - just to show how much more flexible a synthesizer is when creating your own individual sounds and clusters.
So, I think as long as arrangers have this simplified architecture (which has its merits, of course, for those who need it simple and fast), pro band players who want to be creative 'sound inventors' won't buy an arranger.