Fundamentally, we have seen that the isolated Buck requires negative current flow to transfer energy from the primary to the secondary. Almost all standard bucks are designed to support pre-bias operation which means negative current flow is disabled during the startup phase. This prevents the secondary from ramping up along with the primary since sufficient energy is not transferred.
Similarly standard Bucks have blanking times optimized for Inductor applications. isolated Bucks on the other hand involves driving a transformer which has leakage inductance and parasitic winding capacitance, thus making the current oscillations last longer. Therefore, it is not possible to use a standard buck in isolated Buck configuration.