The schedule for JQ flights in and out of TSV are Tue, Thur, and Sun. If the return date of travel was for the Monday there are no JQ flights on that day. If you were unable to purchase a seat in the days either side of this it is because JQ are booked out over the Easter school holidays and these seats were sold weeks if not months before. I'm surprised anyone had seats available on flights over the Easter weekend at all.
There are QF seats available on certain flights throughout the week depending on how much you want to spend.
Fares for SYD-TSV on JQ are on the website as $99 one way Jetsaver and $379 one way Jetflex. For your comparison for the same dates Virgin's cheapest fares are one way $159 up and $109 back.
And yes Denny QF are on a winner with JQ because as much as people like yourself choose not to fly JQ there are a million that will, that's why they are sold out.