Why was it so difficult to unify Italy?

There were at least two reasons why it was not easy to unify Italy.  One of the reasons was internal to Italy while the other was external.

The external reason was that there were foreign countries who had interests in the Italian peninsula.  It was not simply a matter of getting various Italian states to agree to unify.  Instead, there were the French and the Austrians to contend with.  Both of these powers had interests in Italy and had to be dealt with in the course of the move to unify.

The internal reason is that there was a strong sense of regionalism in what is now Italy.  Even today, there is a great deal of animosity between the northern and the southern parts of Italy.  In the 1860s and 1870s there was even more.  Many Italians were more accustomed to thinking of themselves as citizens of a given small state, not as ethnic Italians.  This made them less likely to be interested in becoming part of a larger Italian state.

For these two main reasons, it was hard to unify Italy.