If both premises are particular, they may concern different members of the middle class. Nothing forces the instances selected by the two premises to coincide.
Example:
Some students are Qur'an memorizers.
Some students are calligraphers.
Both claims can be true even if the two groups of students are disjoint. “Some memorizers are calligraphers” therefore does not follow.
A universal premise includes every relevant middle-term instance, allowing it to apply to the particular witnesses provided by the other premise.