South Africa are actually a good example. What allowed them to come straight back to international cricket so easily was the strength of their domestic system. It is the training of cricketers at young ages, where it garners the most benefit. Trying to beat a Test side into shape will always have incomparably small effects.
Similarly, Sri Lanka didn't just take up cricket in 1975, they had been developing for decades before that. However, that exact thing could almost be said of Bangladesh, who didn't have a proper domestic competition until they became a Full Member of the ICC.
The ICC's decision making process is best left for them to explain, but the fact is that the respective Test debuts did not exactly come about at the same point in each member's timeline.