Being quick requires a lot more than balls. There are many current Isle of Mann racers that suck at circuit racing. Their huge balls must be a hindrance when it comes to pushing a bike to the absolute limit around a circuit. Road racers balance speed with living to see another day. They're usually not pushing their bikes to the limit that top circuit racers do routinely. Different disciplines within the sport of motorcycle racing.
They are pushing the bikes to the limit, they're just very different bikes to what you use in a circuit race, for example John McGuinness's TT winning Fireblade is a lot different to the one Sylvain Guintoli races in WSBK as it's built specifically to be raced around the Isle of Man.
This highlights the difference between a winning circuit bike and a winning road race bike - BMW Showdown: Kiyonari v. M Dunlop - we ride them both! | Bike Social