Because they have extractive political and economic institutions.
These are di¢ cult to change though they can be successfully
challenged and altered during critical junctures.
The roots of modern world inequality lie in the emergence of inclusive
institutions in Britain and the fruits of this - the industrial revolution -
spread to those parts of the world that had similar institutions (settler
colonies) or quickly developed them (Western Europe).
Other parts of the world languished with extractive institutions which
have persisted over time and thus remain poor today.