I suspect that the technical reason might be that the NJP would either be hugely expensive to hire, or - most probably - licencing the original soundtracks would have cost even more as well as being difficult to grant. US-wise for example, Spirited Away is owned by Disney and Princess Mononoke is owned by Miramax (who Disney sold off 3 years ago).
Would presumably have been much cheaper to get someone else in to do it - Ghibli don't seem to be too bothered about letting anyone re-recording their stuff, if the reggae Totoro soundtrack is anything to go by
I think mine's turned up (dreaded red postcard of doom from the Post Office so I have to get up early on a bloody Saturday again), I'll compare the stuff on there to the original ones I have on CD here. AFAIK, no Hisaishi scores have appeared on vinyl - someone really needs to rectify this (hello, interested labels who frequent/own this site!) by putting out everything from Sonatine to Dolls, as his work with Kitano is unparalleled.