I’m pretty sure it has to do with baiting people in to watch the rest of the card and buy the ppv, because TV fight events have a tendency to get the primary viewers tuning in towards the end, right before the main event.
I remember a couple of good fights that were put on fight pass prelims in the past, and I heard the reasoning was to attract more people to fight pass.
I mean it’s not too weird when you think about it, a band at a concert usually starts the show with a popular hit right? An album usually starts with a good song too.