Why can't we defibrillate the heart within 1 minute after ventricular fibrillation by electroshock?
To say that there is a cutoff of one minute at which time shocking becomes ineffective is incorrect. The best way to defibrillate a heart within one minute of VFib onset is with electric shock. In fact, short of certain drugs in certain instances, it's the only way to defibrillate Vfib, regardless of the amount of time that has passed.

If the one minute cut-off were true, survival rates of out-of-hospital arrests would be more dismal than they are. Even with the availability of automated external defibrillators (AEDs) to treat out-of-hospital arrest, the collapse to response time is well over a minute. Someone has to notice a collapse, feel for a pulse, and call for a defibrillator (or get it themselves), expose the chest, apply the paddles, wait for the monitor to display the rhythm, clear the area and shock. If you were told that this usually occurs in under a minute, you were misinformed.

Seattle is the best city in the US (probably the world) in which to have a heart attack.[1] The population has a high rate of training in CPR, a highly trained corps of paramedics and EMTs, and a quick response time. So it's logical to do studies about factors affecting survival of out-of-hospital arrests in Seattle.
A study published in 1999 found that CPR did not have any effect on outcomes of out-of-hospital arrests unless the interval time (from arrest to defibrillation) was greater than 4 minutes.[2] (In other words, shocking without CPR was no different in outcome than shocking preceded by CPR; both resulted in cardioversion to a stable rhythm and a return of of spontaneous circulation in the same percentage of patients.)

However, that is not to say that CPR (chest compressions +/- respirations) are unimportant. A 2003 study found an unacceptable level of CPR was performed when AED paddles were connected (i.e. there was an over-reliance on shocking).[3] 

The new Advanced Cardiac Life Support guidelines call for CPR to be initiated immediately, regardless of witnessed or unwitnessed arrest (i.e. down time).[4] It needs to be noted, though, that this recommendation does not rest on defibrillation with the aim of return of spontaneous circulation(ROSC), but on improved neurological outcomes.[5] This, not ROSC, is what the new ACLS recommendations are based on, because ACLS is not considered highly successful if the patient leaves the hospital only to be hospitalized elsewhere in a persistent vegetative state.

[1] King County may be the best place to have a heart attack
[2] Influence of Cardiopulmonary Resuscitation Prior to Defibrillation in Patients With Out-of-Hospital Ventricular Fibrillation
[3] Interruption of cardiopulmonary resuscitation with the use of the automated external defibrillator in out-of-hospital cardiac arrest
[4] VF/Pulseless VT
[5] Delaying Defibrillation to Give Basic Cardiopulmonary Resuscitation to Patients With Out-of-Hospital Ventricular Fibrillation A Randomized Trial