Because mostly lithium manganese mixes is not as stable near 3.2 volts a cell under a 20c load as the lithium iron cobalt, people had shorter flights and the batteries wouldn't hold voltage or last more than 10 flights before the internal resistance of the cells went very high...basically.

The manganese is 6,000 dollars a ton where the lithium iron cobalt (or it's variants) are 50,000 a ton a couple of years back.

Also, with cobalt ... my understanding... since it's more stable near the edges of capacity and voltages ranges they can go higher in volts without adding cells... so there's more power output without having more weight.

Hobby King sells the "bolt" packs and Revo sells a 4.27 packs... more output and more stable

Something the current crop of 30 + lbs fans can put to use