They didn't unite because they were backward, shortsighted low IQ Somali men who were all hellbent on using their own clans to achieve power.
"The SSDF tried to ally with the SNM in 1983, but they failed to agree to a common strategy."
Imagine if they united, the 88 massacre wouldn't have happened and Morgon would have been killed. Majority of the SSDF generals hated him cuz he ruined the 1978 coup attempt by Majeerteen.
"Siad Barre apparently resented the [Majeerteen] clan's prominence, and as early as 1970 was singling out the Majeerteen lineages for alleged opposition to his reform efforts. As a clan, the Majeerteen probably did not oppose Siad Barre at the outset. However, his insensitive rhetoric and discriminatory appointment and promotion policies had the effect, by the mid-1970s, of alienating the heads of the leading Majeerteen lineages, the very persons whose attitudes were decisive in determining the clan's political orientation.
Majeerteen officers were the primary organ of years of an unsuccessful coup in April 1978, following the army's humiliating defeat in the Ogaden War. An estimated 500 rebel soldiers were killed in fighting with forces loyal to Siad Barre, and subsequently seventeen officers, all but one of them Majeerteen, were executed