Because the u in usually is pronounced like the consonant y, like “yew”. We use “a” before consonant sounds.

The u in unusually is pronounced like the vowel sound u, like “uh”. We use “an” before vowel sounds.

This is because two vowel sounds in a row is somewhat awkward and doesn’t flow as easily with how you move your mouth to pronounce things. Using “an” puts an extra consonant sound in between the two vowel sounds.