The reason given on the IMSA stream for the faster GT cars is down to the fact that prototypes never get fast enough to get the aero into the picture which is where most of their speed is. Most of the prototypes aren't much faster than the GT cars in a straight line as you can really see on the oval section of Daytona. Last year's Petit Le Mans saw a GTLM Porsche finish first overall as a result when the entire race was inundated in torrential rain.