By design, your class dictates which skills allow you to reach Legendary proficiency.
Also by design, every class can only reach Legendary proficiency in a maximum of 3 different skills, at 15th, 17th, and 19th level.
Currently, Wizards get access to a mere two Signature class skills: Arcana and Craft. Outside of these, only Lore may be advanced to Legendary.
Note that no other class receives as few Signature skills as a Wizard, and only Alchemists get as few trained skills (and are also coincidentally an Intelligence class too).
Alchemist: 3 (2+Int trained)
Barbarian: 3 (3+Int trained)
Bard: 7 (7+Int trained)
Cleric: 5 (5+Int trained)
Druid: 4 (4+Int trained)
Fighter: 3 (3+Int trained)
Monk: 3 (3+Int trained)
Paladin: 4 (4+Int trained)
Ranger: 6 (6+Int trained)
Rogue: 10 (10+Int trained)
Sorcerer: 5 (5+Int trained)
Wizard: 2 (2+Int trained)
So it would appear that, for a 19th level Wizard, they have the dubious joy of reaching Legendary in Arcana, Craft and a single type of Lore, with zero other options within their reach outside of multiclassing.