Prolly cause 750 cc is all you need. Harley can make a 1000cc bike, but getting power to the ground is more than just horsepower. It's power delivery, chassis, and tires all working together to get that drive off the corners. It's one of the few forms of racing that you can't win by throwing more money and horsepower at the bike.
Sorry, but it's not THAT impressive. SInce the AMA legislated the HOnda RV750s out of the field, AMA Class C Flattrack has been pretty much a Harley-only party, so of COURSE HD's are gonna finnish 1-2-3...
I can't think of a time in the last 20 years when HD's WEREN'T dominating the top 5
Maybe you need to keep up on the rules. 1000cc twins are now legal, so anyone can run any twin up to 1000cc with no restrictions. If Honda wanted to bring back their fantastic RV750, they are free to do so. My guess is they would get their ass handed to them.
I raced at Carrell speedway half mile flat track in 1951 & 52 - the clay track surface was used at Ascot after the 110 freeway was built through Carrell. I also ran Riverside Box Springs TT and Anza Riverside TT as well as Lincoln Park 1/4 mile and Culver City 1/4 mile. There used to be a lot of tracks around LA. A point-to-point hare & hound was run through Monterey Park around 1949 or 50 - it was about 50 miles and only crossed two streets (Atlantic and Garfield Blvds). Also, Monterey Park sponsored a TT race around that time and Ed Kretz won. Lincoln Park was used for speedway racing after WW II until about '51 and I saw some guys like Manuel Trujillo there when I was a teen ager. I believe the Milne brothers and Lammy Lamereaux may have put in an appearance, but I don't remember seeing them. We also ran Scrambles at Lincoln Park, Rolling Hills, and Tarzana. I don't think those days are coming back any time soon.