The Piedmont was the strongest Italian state in Italy in the 19th century. Other states like Venice or Lombardy was under Austrian rule, the Papal States were controlled by the "country" that we now know as the Vatican, the kingdom of the Two Sicilies, though technically Italian, were ruled by a French-Spanish monarch and tied to Spain. So Italian revolutionaries like Garibaldi rallied to the Piedmont, and not to the Vatican or Naples-Sicily.
One very important advantage of the Piedmont was its proximity to France, and its ability to get French help, when the French ruler, Napoleon III would rather have a united but weak and friendly Italian "buffer state" than divide "Italy" with the Hapbburgs. For the price of Savoy and Nice, Piedmont got all of Italy. Other, previous, French leaders had opposed Italian unification and allied with other great powers to keep Italy divided.
It's like asking, why did Prussia unite Germany even though Austria was (initially) stronger. The answer was that Austria had so many non-German commitments that Prussia was seen as a more natural leader. Ditto for Piedmont versus the others.