Question

why is base base band signal not transmitted directly. give two reasons.

Answer 1

Base band signal is not transmitted directly for two reasons : 
 i) Because of the low frequency of base band signals, the size of antenna used should be huge which is practically not possible.
ii) Despite of interferences occured by the waves, the bandwidth of baseband signals tends to infinity and as a result of which a transmission medium's bandwidth must be equally infinite which is vague.

Therefore because of these two reasons baseband signals aren't transmitted directly.