Why does the inhibition of translation initiation cause the accumulation of 80S ribosomal monosomes?
It will depend on the drug you are using. There is a pathway that forms the translational initiation complex that starts with binding the mRNA's cap, the small subunit scans to find the AUG, then the large subunit binds, and so on. The 80 S peaks on the density gradients are not vacant, they are poised on mRNAs waiting to initiate (if you removed the inhibitor).