After 3. ... a5+, Black will quickly run out of checks and will thus be unable to prevent the White checkmate that was the point of the first move. With the solution the puzzle uses, Black wins the White bishop and creates an escape square for the king at h6.
2. Ne7 Ra2+
3. Kb4 Qd2+
4. Bxd2
3. ... a5+
would be more likely played