ADSB is not mandatory in the US below FL100 or FL180 (10000/18000 feet), that covers most helicopter flights.

It depends also on the website you are using to track. I have an ADSB receiver that publishes to multiple tracking websites (the same data, unfiltered), and not all of them publish all the data. Flightradar24 doesn't show most of the military aircraft - I can see them on my local tracking interface but they are not shown on their website.