No doubt issued to the Home Guard for the obvious reason that they were a very good design,
and being a commonly issued military belt, so available in good number from stores also probably didn't hurt.
Another advantage of the 1903 Belt was its very simple & basic design, not having any sewing or riveting in the construction
(aside from the running slider, which could be finished using either of the two methods)
I have a little bit of extra information on the belt design in the link above.
For modern users, I can supply these in a range of sizes, not just the WW2 Small, Large & X-Large options.