Was nothing to do with backhaul, but mainly down to two things.
Originally before ofcom intervened with dragging pricing down, burst speed was priced at a premium on DSL ports at the exchange.
So if you wanted more speed you paid more, I actually preferred this personally as I considered it fairer, if your line synced poorly e.g. you buy a lower end product and pay less. It wasnt just a few pence either, the difference between 0.5mbit and 2mbit was significant in price.
The second primary reason was the accepted mindset of selling DSL, which was to provide a fixed speed rather than a "up to" lottery speed.
However ofcom were not happy, they wanted high speeds sold for peanuts which led to a revamp of wholesale pricing and the introduction of adsl max, a service where speed was anything from a couple of hundred kbits up to 8mbit/sec. A product I hated.
This was probably the worst era for UK dsl as well, as the wholesale revamp was brutal, BT were pretty much forced to sell high burst speed for pennies, but to compensate them they were allowed to massively overcharge for backhaul so their revenue stream was protected, this led to a situation where isp's massively oversold capacity as the cost of supplying backhaul at proper capacity was too high to keep their product competitively priced, this was one reason how LLU got so popular as it was far cheaper to get backhaul via LLU then from BT wholesale.
Some people even considered burst speed to be a free commodity and BT's backhaul pricing to be a market standard due to this artificial arrangement it was sad, but thankfully its now history to a degree. We still have "up to" on FTTC and g.fast but at least its not one big product as "up to" from 200kbit to 330mbit priced at a single price point.