Are you joking?
The whole point was that white athletes in Europe were more competitive because they had greater access to PEDs than black athletes in Western countries. Notice how few whites were competitive times wise outside of Europe (ie in the US). Even though Allan Wells is great and all, he still ran times not even close to the WR at the time. It goes without saying that many black athletes were not afforded many opportunities until the 70s and you would expect that it would take at least a decade or so (probably more like 20 years or so) for the results of such opportunities given.
There are positive tests now because they can actually test for the drugs. There was no random testing before 1989 and minimal effective testing at all before 1983. It goes without saying that many Eastern European countries controlled the testing procedures in the first place, which makes it unlikely you would see a positive test.