choral scores include the other parts, but orchestral scores rarely include other parts

For one thing, there are rarely more than eight choral parts and usually only four. If the orchestral scores had everybody's parts it would be dang near impossible for anyone to pick out the individual part, not to mention terribly unwieldy for someone whose hands are not free as are a singer's. It is common practice among singers to highlight one's own line, particularly if there are divisi scattered about here and there. Early practice in choral scoring was to include only the part being sung. The present practice may be as much for the convenience of the publisher as for the convenience of the singer; they only have to print one version.