It's dependent upon the technology used. For example, fiber optic to the home (like Verizon's FiOS...FTMFW) is dedicated bandwidth. You'll always get the speeds advertised. Cable, on the other hand, shares the overall bandwidth between the central office and the individual user, resulting in decreased throughput during high usage times.
Just primarily due to inherent limitations in the media/technology used.
And when I move, I have to give up my FiOS. My choices are either cable or dsl at my new place. Kind of like having to choose which turd to eat: the one with corn or the one with peanuts.
“Everybody is a genius. But if you judge a fish by its ability to climb a tree, it will live its whole life believing that it is stupid.”
― Albert Einstein
Doing the popular thing is not always right. Doing the right thing is not always popular