There's a roundup of some previous answers regarding WW2 in this thread. I could swear I saw an even better response more recently, but haven't been able to find it. The short version, for WW2, is that the country lacked the industrial base to properly equip modern armed forces, and also suffered poor leadership, in part due to a poorly educated population.

I haven't seen a discussion of Italy in WW1 on here - did they perform so badly compared to other belligerents? - but the country's low industrial base would undoubtedly have had an effect there as well.