I'd hazard a guess one reason is preventing users changing things so BT know exactly the configuration of each modem they hand out on each line - and the modems are matched pairs to the DSLAM's type fitted in the particular fibre cabinet that feeds your property.
They would want the unit to be regarded as a "black box" that simply works and you don't need to bother about it and how it works/what it is doing - in much the same way that your car's ECU just works and I've never had any access into it or cared that I did not.
Most people are simply not bothered, indeed most people are not interested in any of the details about broadband whatsoever and anyway have no ability to understand them. Actually round here no one is even bothered whether fibre arrives or not - I know I've conducted surveys round the village and no one is really interested despite all the hype in the press about FTTC broadband being "essential".
It is a very small minority of the UK BB users on ADSL/FTTC who want access to the modem's stats.
