As we know, in IEEE 754 standard, float number's exponent and mantissa are stored differently. While the exponent is stored as an unsigned number, taking advantage of the bias, the mantissa is in sign-magnitude format.
The reason to use a bias seems to be the fact that another value of exponent can be achieved this way, effectively providing a considerable amount of "new" numbers that can be represented (2^24 for 32 bit floats).
On the other hand, using the same system in mantissa would provide only 2^8 new values which, given the whole range of a 32 bit number, is nothing.
For humans, it would obviously be much more difficult to read float numbers with mantissa expressed the same way the exponent is.
Is that the sole reason against such notation?
Isn't a unified notation system, simplified arithmetics and a tiny bit larger range worth the hassle?