For the same reason Germany, Japan, Italy, France, and Japan made no serious attempt at rearming with semi-automatic rifles. Admittedly, Germany did make an effort to manufacture the STG44 late in the war, but it never became nearly ubiquitous. Basically, a semi-automatic rifle required significant industrial development (R&D, but also retooling) for a weapon that offered marginally increased effectiveness at the cost of a more complicated, fragile, and expensive to produce design.

In Britain's case in particular, the immediate pre-war years were a time of military famine. Defense spending had been cut throughout the 1920s and 1930s, and when rearmament began in earnest, ships, airplanes, and tanks were more in demand. They had a very effective (perhaps the most effective) bolt-action rifle in the Lee-Enfield, and it was simply easier to modernize it than to invest in an entirely new design.