Danny, the Champion of the World was written in 1975, a time when it was very unusual to write about a father raising a child as a single parent. As Jeremy Treglown (Roald Dahl, A biography) comments, "It is an ode to a newly-prevalant condition".
In 1971, only one in thirteen families was headed by a single parent. By 2004, one in four dependent children lived in a lone-parent family. Most lone parent families are headed by the mother, with fathers representing only around 10% of all lone parents. Dahl was writing about a very rare phenomenon for his time.
Roald Dahl's own father had died when he was only three. William, Danny's father, appears to be an idealised version of the father Dahl would have liked for himself.