Both Canada and the US have long histories in rugby culture. In fact the most recent Rugby World Cup was the first world cup not to feature either team. Canada had been at every WC since the inception in 1987 and the US had been at all but one (1995).
On the women's side, both teams have a strong 15s team. The US were in fact the first ever women's World Cup winners and Canada have been runners up. In fact Canada are currently the 3rd ranked side in women's rugby, behind only England and New Zealand.
Domestically, the US has been playing rugby since the late 1800s, primarily in universities, which is where the majority of North American rugby players find the sport, on both the men's and women's side. In the last 20 years it has received a surge in popularity and is regularly cited as one of the fastest growing sports in the US. In fact the 2031 Men's World Cup is going to be held in the States, with the women's WC in 2033 also hosted there.
Women's rugby has historically received little investment and support, even in the countries where it is popular. That fact is similar to women's football until relatively recently.
World Rugby has started showing support for the women's side more seriously in recent years and, sure enough, the game is growing at pace. The addition of 7s to the Olympics for both men and women is part of that.
Incidentally, the 7s this week set a record for a stand alone women's rugby tournament, with an attendance of 66000 people.
So it's not 'just there for gender equity reasons". It's there because people enjoy it.