Why did Cai Lun invent paper?

In ancient times writings and inscriptions were generally made on tablets of bamboo or on pieces of silk called chih. But silk being costly and bamboo heavy, they were not convenient to use. Tshai Lun [Cai Lun, Tsai Lun] then initiated the idea of making paper from the bark of trees, remnants of hemp, rags of cloth, and fishing nets. He submitted the process to the emperor in the first year of Yuan-Hsing [+105] and received praise for his ability. From this time on, paper has been in use everywhere and is sometimes referred to as 'the paper of Marquis Tshai'.