When it was introduced - and it was new at the time - not everything was as neat and clear as you state.
Quoting Wikipedia: "The [TCAS] manual described TCAS as "a backup to the ATC system", which could be wrongly interpreted to mean that ATC instructions have higher priority."
In fact, you'd do well to read the entire Wikipedia article to see that this was not the only case where pilots did not have clear guidance on which orders to follow in the case of conflicting ATC and TCAS orders.
Quoting Wikipedia: "The [TCAS] manual described TCAS as "a backup to the ATC system", which could be wrongly interpreted to mean that ATC instructions have higher priority."
In fact, you'd do well to read the entire Wikipedia article to see that this was not the only case where pilots did not have clear guidance on which orders to follow in the case of conflicting ATC and TCAS orders.