Death is a very intense, a very strong word. It highlights both our helplessness and the urgent and crucial necessity for us to experience the life-giving power of God. If this helplessness was not spiritual, then perhaps salvation might come through something other than power. For example, it might come by wise counsel or by powerful exhortation. But the hard facts disallow the effectiveness of such a strategy altogether - it would be quite useless.
Death puts men beyond the reach of good advice.
It is sheer folly to ask a dead person to do anything. That would be asking for the impossible. It would be asking for a definitive, positive course of action in the face of complete inability to respond. In the matter of salvation it is exactly the same. The sinner does not only need good advice or reasonable exhortation in order to become a Christian.