It has to do with the technology the ISP is using to bring the signal to your home.
For instance, ADSL2+ can reach 24Mbps but you have to be in a 2Km radius from the DSLAM, the equipmant of the ISP in your local "station" (don't know the right term for this). The more far you are from the DSLAM lower is the speed you get. For everyone to get 24Mbps the ISP has to install a lot of DSLAMs so the clients be in the 2Km radius. In a big city this investment pays off because the clients are very close to each other, but if you live in a remote area where the houses are far from each others this investment doesn't pay off.
On the other hand fiber optics can reach a lot more, if I'm not mistaken we don't know the theoretical limit of the fiber optic, and for residential access it can reach up to 15Km, using GPON technology. The limitation is in the equipment, the amount of information it can process and transmit to the fiber optic. And install FO to your house isn't cheap.
Notice that when you subscribe for a 24Mbps access the ISP only guaranties that speed between your house and the DSLAM. Your speed will be limited also by the uplink speed of the DSLAM to the backbone network. Here in Portugal the rule is 34Mbps or 155Mbps uplink for each DSLAM. This means if the DSLAM has a 155Mbps uplink and 20 people with a 24Mbps subscription start to download a file at the same time they won't reach their maximum speed, only 155/20, approximately 8Mbps. This is true for every technology used.
You can be capped not in the DSLAM uplink but in the link between ISPs, these links cost a lot of money so the ISP doesn't rent a link with the total capacity needed if all their clients decide to request an object from the same ISP. They do some math and rent a percentage only, so if there are more customers that they predicted requesting information from another ISP then you won't have the full bandwidth you subscribed from the ISP.
Hope this isn't very confused but I don't have much time to write and my english isn't that good