The straightforward algorithm for addition is more complicated than the straightforward algorithm for multiplication. For addition, you need to compare the exponents, shift the mantissa of the operand with smaller exponent, add or subtract (the signs might be different) and then check for overflow or check for an unnormalised result. Lots of complicated steps.

For multiplication, you multiply the mantissas, normalise the product of mantissas, check for overflow and under flow. Much simpler. 

However, the number of operations for the multiplication is much higher, quadratic for typical fast hardware implementations. It’s just easier to describe.