Important conclusions from the Rutherford experiment were drawn from the number and angle of reflection of the helium ions from the incident angle. Returning back of approximately one in twenty thousand incident alpha particles due to repulsion confirmed the presence of a heavy and large positive charge in the gold atom (Z =79).
A foil of small atoms like aluminum, having a very less number of positive charges (Z=13) will not be able to repulse a fast moving alpha particle to return. At the maximum it may deflect them at small angles.
Hence, the return of alpha particles would not have been observed and the establishment a nucleus would not have been made.