Monopoly firm's block pricing strategy making max. profit
The reason why the profit maximising condition is like that is because the monopoly is Price Discriminating between those who are willing to pay at  $P_1$ and $P_2$ for $Q_1$ and $Q_2$ respectively.

This increases the area of producer surplus to an area greater than what it would be by charging $P_1$ and $P_2$ alone.