The imperial powers, mostly Britain and France, with a little help from Belgium , Germany, Italy and Portugal, brought infrastructure and services to the African nations they colonized. Roads, bridges, electricity, a basic health system, schools (albeit for English, French and religion). Later, these countries would need these basics when they achieved independence, and Rhodesia was no exception.
The British tied up all of the arable farmland in the hands of a few rich whites, which kept native Rhodesians locked into poverty. They benefited little from the education system, and lacked the skills or experience to run their future nation.
This is, in general, the same answer as you could give for many other countries. British colonization helped set up a pretty decent economy at the time that Rhodesia became Zimbabwe, but it did not really get the country ready for independence.
At the time that Zimbabwe became independent, it had an economy that was one of the strongest in Sub-Saharan Africa. But the economy was based largely on white farmers and other whites. The black majority had not been given the resources (educational, etc) to really take a large part in the economy. You can see this now falling apart as whites flee the country.
Similarly, the population as a whole had not been allowed to take much part in politics. This left them open to exploitation by a dicator (Mugabe).