Vita simply lacked a good library of games. If it had gotten the same first party support Nintendo gave the 3DS it would have sold better.
Switch sells because it keeps getting good game after good game. This year we'll still get Steamworld Dig 2, Pokkén Tournament, Super Mario Odyssey, Doom, Xenoblade Chronicles 2, Skyrim: Special Edition, Fifa 18, Axiom Verge, Battle Chasers: Nightwar, Thimbleweed Park, Rime, L.A. Noire, Resident Evil Revelations 1 & 2, Wulverblade, Golf Story, Rocket League, etc.
Switch has everything: big, small and medium-sized games.
In its first year on the market the Vita had 16 games that scored over 80% on metacritic. The Switch currently already has 21 and there's still 3.5 months to go.
"The strong do what they can and the weak suffer what they must" - Thoukydides
