Among the factors: Uneven basketball development opportunities in Canada; National Collegiate Athletic Association requirements that make it difficult to qualify for Division I scholarships from Canadian high schools; and a desire on the part of some players to find safer and more stable places to play and study than they have at home.
"Most of my friends [in Canada]are basketball players," Thompson says. "And they're like, I'm out of here too, let's go make it happen."
The emigration of Canadian talent - some feel the current cohort of players from Canada is the deepest and richest ever - has become a self-fulfilling prophecy. "Welcome to beautiful Tampa, FL where we hope to see you enroll," is the pitch on one basketball message board by a coach looking to import talent for a start-up program where tuition is $14,000 (all currency U.S.) a year. "WE NEED PLAYERS."
Ro Russell, a fixture on the Toronto basketball scene through his successful club program, Grassroots Canada Elite, took a bold step this year when he moved to Creedmoor, N.C., to start an elite team affiliated with Christian Faith Center, an evangelical church.
"I'm the most pro-Canadian guy there is," says Russell, whose roster is nearly entirely made up of kids from the Toronto area. "But for basketball, the proving ground is America."
Many of the kids heading south are from immigrant minorities, from tough neighbourhoods and less-than-ideal home situations.
"There has been a lot of crime in some of those neighbourhoods, no doubt about it," says Mike George, founder of CIA Bounce, another top Toronto-area club that has had several kids head south for high school. "For some parents, they see it like sending their kids to boarding school, it gives them a sense of security. I've had kids get full scholarships to [elite East Coast private schools]Proctor or Brewster. That's worth $45,000 a year. For them it's a no-brainer."
Not all kids are so fortunate. It's estimated about 80 per cent of them have to pay some or all of their tuition, with only the elite getting scholarships to prep schools.
Then there are rumours of kids sleeping three or more to a room, 10 to a house, with minimal adult supervision, essentially fending for themselves at schools with dubious academic standing and scant record of developing elite basketball players.
Thompson and Joseph have had the opposite experience. They share a 3,000-square-foot house with their teammates a short walk to the campus of Henderson International, their expenses covered by Cliff Findlay, a wealthy Las Vegas auto dealer who underwrites the entire operation, including the salaries of two full-time coaches.
Both come from good families and neither of them were at risk of remaining undiscovered had they stayed home. Each starred on the summer Amateur Athletic Union circuit, where reputations are made and recruiting relationships often start. They've also represented Canada internationally. And players of Thompson's size, athleticism and academic chops - he can fill your ear about the role of female characters in Shakespeare almost as well as he can face up, spin baseline and dunk in traffic - don't go under the radar. He's had clips on YouTube since he was 15.
Joseph, who carries a 3.2 (out of 4.0) grade point average along with a silky jumper, knows staying home for high school isn't a barrier to entry for Division I basketball. His brother Devoe went from Pickering High School to the University of Minnesota.
But as an elite big man, Thompson wanted to test himself against other top bigs. Joseph needed to upgrade academically. Both have succeeded beyond their expectations.
But it's clear each of them craves something he wasn't getting at home. Like aspiring NHL players from the U.S. who come to Canada for major junior, there's something to be said about being in a place where what they care about most is cared about deeply.
"The exposure, the hype, basically the hype," Joseph says. "In Canada, not many reporters come to games, not many D1 coaches. Here the media are at the games, the college coaches, the fans. It's like [the provincial] championship every game."