Why

Mubutu

Zaire was a dictatorship, the leader, Mubutu Sese Soko, wanted to use football to give him and his country a better name internationally. He spent a lot of money on good training facilities and hired a famous coach. Zaire won the Africa Cup and Mulumba Ndaye was the top scorer. Then they went to the World Cup.

afraid

The players were afraid of the dictator who had even told them not to come back if they didn't win the Africa Cup, a serious threat.
But they weren't too afraid to complain to Mubutu about the minister of sports who had pocketed the money the players should have received from FIFA (about a half a million dollars). Mubutu promised to do something about it. He sent someone to Germany to tell the minister to give it back. The minister did give the money, but not to the players but to Mubutu's man who immediately took off with it. Ndaye says the money ended in Mubutu's pocket.

so

The Zaire players didn't want to play anymore, but when they told FIFA they were quitting, FIFA ordered them to play or receive a 5 year ban. Too afraid to tell the world what had happened with the money and too afraid to abandon the match they stepped on the field, a broken team. And when the referee sent off their best player for a foul another player had committed, they probably couldn't care less about the result.


source:Edwin Schoon De macht van de bal

but

People remembering this match say Zaire tried really hard the first 20 minutes of the match. They were the much lesser team, less well organized, less well trained etc. Not being paid wasn't an exception for African teams in those days (still isn't), Zaire just wasn't good enough to play a European side, they say.