His parkisons has been noted as having become severe enough to inhibit his ability to do voice over work. If he did it, it would be difficult and very time consuming. If you look at his imdb page you'll see that while he has been active in spurts there also tends to be lulls in his career in the last 10+ years.
Its really a shame too because I'd love it if he could do the voice. (Heck I'd love a classic Spin-City based adventure game.. >.>)