Here's why:
* The subject ("the president") is being acted upon. The president is not the one doing the action of developing the economy.
* The action ("develop") is performed by an unspecified agent. We don't know who or what is actually doing the developing.
To change it to active voice, you would need to specify who is doing the developing. For example:
* The president was trying to develop his economy. (Passive)
* The president was working to develop his economy. (Active - implies the president is doing the developing)
* The advisors were helping the president to develop his economy. (Active - specifies who is doing the developing)