Why do scientists think it is necessary to develop new ways to create ethanol

Due to the cost and resource input (specially the land) required for producing ethanol

Explanation:

In current time most of the ethanol is produced by fermenting corn, sugarcane or other plants at high temperature. In this process of fermentation, liquid bio fuel is produced.  

These plants and crops require large area for cultivation and production along with huge inputs of fertilizers and water. As per an estimate , 3 gallons of ethanol require bushel of corn that intakes 800 gallons of water which is a huge cost to any nation or individual  

Hence, scientist are  finding new ways to create ethanol