It's just the reviews on such low cost phones runing FFoS are pretty bad, and once you climb to $65 or $100 , windows phone and android offer much better alternatives.
Most of these reviews are unfair (comparing a 25$ FxOS to devices at 2x the cost or more) but you make a point about FxOS not being attractive for higher-end. That's part of why it's currently being reinvented.