Consider a fixed point representation which can be regarded as a degenerate case of a floating number. It is entirely possible to use 2's complement for negative numbers. But why is a sign bit necessary for floating point numbers, shouldn't mantissa bits be using 2's complements?
Also why do the exponent bits use a bias instead of a signed-magnitude representation (similar to the mantissa bits) or 2's complement representation?
Update: Sorry if I didn't make it clear. I was looking for the reason of how floating point representation is shaped. If there is no strong implementation trade-off between the alternatives, then could someone explain the historical aspects of the floating point representation?