Rhodesia was treated more harshly. This was a combination of it being the progressive/left-wing 1970s, as opposed to the conservative, right-wing 80s (noted apartheid apologists included Margaret Thatcher). There was probably a greater willingness in the 70s to believe the Africans could govern themselves, which was less widely believed in the late 80s, in Western countries. South Africa was also much more powerful and economically influential than Rhodesia, as well as being more clever in terms of PR. Another consideration was the African case was compromised by intense in-fighting and division, between the ANC and the Zulu-led group of Mangosuthu Buthelezi.

May I ask where you heard this? Unless I am woefully mistaken South Africa was treated like a pariah for a long period of its history, and while Wikipedia is hardly a good source a quick look at a couple of the pages show that there were large boycotts of Apartheid South Africa.

In part because Rhodesia declared independence unilaterally without the consent of the UK, the government of which was insistent on majority rule as a precondition to independence. Thus, Rhodesia never had international legitimacy to begin with, unlike South Africa which became a dominion of the British empire in 1910 and became fully independent as a consequence of the Statute of Westminster in 1931, which made the British dominions (South Africa as well as Canada, Australia, New Zealand, and Newfoundland) independent. At that time, there was far less attention paid to the plight of the native black Africans than there was in later decades.