Because handhelds have weaker specs, and Nintendo only had one serious contender ever in the handheld market. That of course was the PSP. And only the Switch Lite is keeping dedicated handhelds alive.
Lifetime Sales Predictions
PS4: 130 mil (was 100 million) Xbox One: 55 mil (was 50 mil) Switch: 110 million (was 73, then 96 million)
3DS: 75.5 mil (was 73, then 77 million)
"Let go your earthly tether, enter the void, empty and become wind." - Guru Laghima