If a big storage capacitor is used with power supply, then not only discharging lasts long, but also charging! If this is the case, your 0.1µF capacitance at reset circuit is much much to small. Time constant of reset circuit must be greater than rise time of Vcc, otherwise you won't get any useful reset signal!
So, choose 4.7µF capacitance at reset circuit and look what happens.

By the way, these RC reset circuits do not provide a reset signal when supply voltage goes down. This is a big mess with flash micros, because some manufacturers state, that a missing reset at power down can cause the code memory to be corrupted!! So, it's no good idea at all to use this simple RC reset circuit with your AT89C2051. Choose a specialised reset controller and you will be fine.

Kai