Historical biases are a big factor. English literature has its roots in a society with a lot of class and race divides. The upper classes were more likely to be literate and have the resources to write novels. So, their experiences and the experiences of those like them were more likely to be the focus. Also, the idea of what is 'universal' in literature has been skewed towards the experiences of the dominant groups. So, groups that don't fit into that dominant mold are rarely the protagonists in the English novel.
Stereotypes play a part in why certain groups are rarely the protagonists. There are often preconceived notions about what makes a 'good' or 'interesting' protagonist. For instance, the idea that a strong, male hero is the ideal protagonist has been prevalent for a long time. This has excluded groups like women, who were for a long time relegated to secondary roles. Also, lack of representation in the literary world itself means that there are fewer writers from certain groups who can tell their own stories as protagonists in English novels.