Power density is much lower. On a vehicle you have to pedal one the power is out, weight savings matters.

Having to pedal without power from battery is extremely rare on e-bikes. They have range of 100km+ and few people bike that much between charges.

LFP:s are less popular for DYI because you need a much smarter BMS to deal with the flat voltage curve when charging. A cheap, easily available open source LFP BMS should solve that.

You don’t need a “much smarter” BMS for LFP, Daly BMS for LiPo and LFP are roughly the same price.

For SoC many ebike BMS just use voltage curve to give a rough approximation. But with LFP you need a shunt and a coulomb counter, because the voltage curve is essentially flat until the very last Wh.

I ended up pedaling without power on a 3 hour trip in the mountains on a 750Wh battery/85Nm motor. Can be easily done ;-)