Does anyone know why this place was closed down? I used to live right by it and had always wondered what the reason was... maybe funding?

no idea actually....there was a big mold problem in the basement (when I went anyways), but otherwise it looked to be in good shape. Perhaps the scores of developed lands all around made it useless to have a restaurant / business center / gas station there...although the 401 is so busy, I'd find that hard to believe

From what I understand, this closed because of the massive build-up around it.

Massive build up around it, but also too many other gas stations that be accessed from the off ramp. In addition it was hardly used. It was mainly for people that rv's and trucks, mini vans. And kids that wanted to do the horizontal mombo. Cops never came by, you could sleep there. But it had the cleanest washroom there. At one time, it had servers that would hand you towels, wipe your shoes. It was just weird. It was a great memory as I met my future wife there from a blind date setup on the internet back in 1995.
All it is now with the Shell gas station gone and the building itself, its a large parking lot. Funny the signs are still up, one pointing to parking,and the other for camper parking, the lights are still there as well.