The Swiss didn't particularly want to join Italy, but the Italians had a vague desire to annex Canton Ticino (edit: will source later. Please don't kill me.)

Italian unification was a pretty scrappy affair. Italian-speaking territories like Nizza and Savoy were unscrupulously traded to France for military assistance (and the existence of Corsica, whose natives speak an Italian dialect, was pleasantly ignored). The possibility to annex Dalmatia after WWI (which was historically Italian) was passed over to annex Süd Tirol in order to guarantee a natural border with Austria, ignoring the fact that the entire area is German-speaking.

I'll give a longer answer later, but given that successive Italian governments were too cynical or too incompetent to truly unify the country, there was no plan to annex Ticino, and Swiss Italians showed no desire to join Italy.

Edit 2: Now that I have some more time, I can go into some more detail: While there was a national conversation about the notable exclusions of Italian unification almost immediately after the proclamation of the Kingdom of Italy, the conversation was, similarly to the Italian unification process, spurred by external actors, many in the form of Pro-Italian-Unity political groups, mostly in Austria-Hungary, working towards the diplomatic annexation of territories like Trentino and Istria.

Apart from the general uselessness of these groups (the most successful one was able to keep Italy from losing Trieste after World War II, hardly a victory, seeing as the rest of Istria was lost, with a brutal displacement of the native Italian population to boot) they were less vocal and widespread in the Canton Ticino, because before, during, and after the Italian unification process, the Ticinese liberals won several spectacular electoral victories. The liberals, apart from supporting secularization of society, favoring public education as opposed to the traditional parochial school system, were also naturally sympathetic to Italian unification, eliminating the need of political activist groups to further the cause in Ticino. However, during the Risorgimento, the Austrians very effectively sealed off the border to Ticino, as several pesky Italian Risorgimentisti had sought refuge there, thus limiting contact between Ticinese liberals and the wider Italian Risorgimento.

Another consideration to take, and this is also useful to consider when observing the behavior of the German Swiss during German Unifiation, is that in 1845 Swizerland fought a Civil War (called the Sonderbund), broadly against Protestant-Liberal / Catholic-Conservative lines, with the Protestant-Liberals, many of whom were Swiss-Germans, fighting for a more centralized Swiss state, thus preemptively discouraging union with Germany. The mostly Liberal Ticinese Ruling class, although ruling a devoutly Catholic Canton, naturally sided with the Liberals. Thus, by the time Peidmont had kicked the Austrian Empire out of Italy, Swizerland had adopted a new constitution, centralizing lots of power in the Federal Govornment. Subsequent policies by the Ticinese Liberals to keep the Conservatives out of power following the Sonderbund saw them not only step up their policies of secularization, but asked for the detachment of the Ticinese parishes from the bishop of Como (creating a Bishop in Lugano to which Ticenese pastors would report to, who could be directly influenced), leading the Canton to be further distanced from Italy, which in spite of a strong liberal current among its intellectual class, came out of the Risorgimento as a very conservative and reactionary state.

Further, all these considerations existed almost solely for the intellectual/ruling class. Part of why the Ticinese Liberals were so focused on consolidating power inward (setting aside their pro-unification sympathies) was that they were ruling a fundamentally conservative Canton, whose population was by and large illiterate, staunchly catholic, employed in agriculture, and possibly only vaguely aware of the Italian unification process.