This is a great question. In short, Lithium Iron Phosphate has many distinct advantages over Lithium-Ion, safety being the biggest advantage.
Newcastle Systems has a great write up on the pro’s and con’s of each technology and why the LiFePO4’s are a better choice.
In an nutshell, the phosphate has much better stability which makes it far less likely to catch fire. Combine that with good density and long life expectancy, and the LiFePO4 is a much better battery.