Note this only appplies to the Models AandB as the Master had configuration set in NVRam.
One of the BBC's initial listed requirements for their computer to Acorn was a default of Mode 4 :
" START-UP OPTIONS
A set of internal links will define the input, output and storage channels, and the screen display option, to be used on machine start-up. Default options will be as follows:
Console input - Keyboard.
Console output - VDU display in mode 4.
File input - Cassette recorder, high speed.
File output - Cassette recorder, high speed "
Source ( as well as other sites )
I do realise the tape control speed was moved to an OS command and the links on production machines control the startup mode and disc drive timings.
The question here is simply -why- was the Mode 4 requirement dropped ?
You could say default available memory vs Mode 7 however any program worth it's salt would start by selecting a Mode. A standard Model A does support this mode although memory would be a bit sparse for Basic unless it was changed.
Also if you look around at various keyboard pics which is where the option links or dip switches would live, it seems very few of any at all early models had these fitted. Back then I saw numerous new machines and I do not recall seeing any with the dip switch fitted and none with a default mode other than 7 :)