The probability is he had too little peace of mind to undertake anything that required calm and protracted labour.
"While so many of my acquaintance were married, or in Parliament, or advancing with a rapid step in the various roads of honour or fortune I stood alone, immovable, and insignificant....

The progress and the knowledge of our domestic disorders aggravated my anxiety, and I began to apprehend that in my old age I might be left without the fruits of either industry or inheritance." Perhaps a reasonable apprehension of poverty is more paralysing than the reality.

In the latter case prompt action is so imperatively commanded that the mind has no leisure for the fatal indulgence of regrets; but when indigence seems only imminent, and has not yet arrived, a certain lethargy is apt to be produced out of which only the most practical characters can rouse themselves, and these are not, as a rule, scholars by nature.

We need not be surprised that Gibbon during these years did nothing serious, and postponed undertaking his great work.

The inspiration needed to accomplish such a long and arduous course as it implied could not be kindled in a mind harassed by pecuniary cares.