I've never heard of either of the 2 acts but I will take your word for the relative success of them, but I do have an explanation.
About 20 years ago, I saw Alexander O'Neal play as the opening act for British singer Lorraine Cato here in the UK. Now, I don't know if you know much about this type of music, but at that point, Alexander O'Neal had already had a Gold certification in the US and Platinum in the UK with 700,000 sales of his album "Hearsay".
I can't find the equivalent sales data for Lorraine Cato; she does not have a Wikipedia page.
It had already been fairly common during the 80's for a record company to pay for their act to be the warm-up for an established artist, just as a way of gaining exposure for the act with a group of people who clearly were fans of that type of music.
By the 90's, this state of affairs had extended to the point where the established artist was being paid to be the support act for the new artist, presumably to draw the audience and to add some cache for them by suggesting the new artist was actually the reason the audience was there.
So, in short, the answer to your question is: Cash.