Zimbabwe had been a one party dictatorship under Mugabe for a long while. And those are the official reasons.
The actual reasons are those and the recurring murders and expropriation of white farmers in Zimbabwe. In order to give the land over to Zanu-pf cronies of Mugabe's.
This was extensively covered by international press at the time and anyone could draw their own conclusions as to the relation between that coverage and the imposition of sanctions.
A date-limited search on Google shows that easily enough. For example:
A hit squad has murdered another white farmer, Mr Alan Dunn, in Zimbabwe. Six men dragged him from his home on his farm near Beatrice, 35 miles south of Harare, on Sunday and bludgeoned him with concrete blocks and bricks. In less than 10 minutes Mr Dunn was unconscious in a pool of blood with a fractured skull, two broken arms and internal injuries. He died early yesterday. The political killing provides fresh evidence that President Robert Mugabe is maintaining a campaign of state-sponsored violence against opposition supporters.
Mr Dunn's death brings to 19 the number of backers of the Movement for Democratic Change (MDC) killed by Mr Mugabe's supporters since April 1st. The dead include white farmers, black farm labourers, city residents and peasants.
I.e. sanctions may or may not have been on the horizon, but frequent lurid reports of actual violence against people who may have had family and acquaintances in the UK and other Western countries kept this on the fire until it boiled over and sanctions were instated.
Similar murders have happened in South Africa but they seem more due to general criminal activity, whereas those in Zimbabwe were apparently tolerated by the government and were in a context of expropriations.
p.s. while one could question why white farmers had such large land holdings, it remains true that those were the most productive farms in Zimbabwe by far and that Mugabe's cronies ran them into the ground once they took over, contributing to Zimbabwe's overall woes.
