Broadband means shared network, shared internet. Thats the biggest reason, its cheap. Since broadband is a shared network, speeds are not committed or promised or guaranteed.
There are a lot of internet users, who think their ISP isnt delivering the speeds what they advertise online. Many residential and business broadband internet providers advertise bandwidths up to a certain bandwidth; e.g., their fine print may say “up to 100Mbps down and 20Mbps up”, for example. Of course this means that even 10Mbps down and 1Mbps up satisfies their promise. In the industry, this is often called “Best Effort” service – they attempt to give you that published number and possibly, under the right conditions, they will.
If you want a committed bandwidth, you need go to a Dedicated circuit which has a SLA (Service Level Agreement) promising a certain bandwidth with a penalty to the provider if they do not satisfy their SLA. Of course, such service costs a lot more than Best Effort service.