Although existing answers are not bad, I think they fail to stress the most important reason.
North Cascades National Park is not easily accessible by car.
The only roads in the park are the Stehekin Valley Road, which is only accessible by ferry + foot/bicycle, and the Cascade River Road, a rough, remote, unpaved mountain road that is closed most of the year and that is far out of the way
When we look at Visitor use statistics for backcountry camping use only, the differences with other parks are much smaller. Although it's still 5 times less than Olympic National Park and 3 times less than Mount Rainier Rational Park, that's a much smaller difference than for the total visitor numbers.
Let's have a look at the figures for 5 NW parks, and then one SW park that also lacks vehicle access:
North Cascades National Park overnight use
In 2018, 26,703 backcountry campers. All other categories: 0 (I don't know what happened in 1991, perhaps they closed some vehicle-accessible campground or changed the park boundaries). Some of them are hikers on the long-distance Pacific Crest Trail, which passes through the southern unit of the park.
Olympic National Park overnight use
In 2018, 128,134 backcountry campers.
Mount Rainier National Park overnight use
In 2018, 64,294 backcountry campers.
Ross Lake National Recreation Area overnight use
In 2018, 26,816 backcountry campers.
Glacier National Park overnight use
In 2018, 32,349 backcountry campers.
Channel Islands National Park
In 2018, 21,463 backcountry campers. That's even less than North Cascades national park, for a park from where you can literally see the Los Angeles area. And you don't even need to hike to get there — you can get right there by ferry. But what does it have in common with North Cascades: Not accessible by car.
The United States has a very strong car culture. When a place is not accessible by car, many people will decide to not go there.