Not because Pinochet was not brutal, of course he was... he killed 3,000 people in brutal ways for 17 years. Sure... But add 2 zeroes to that death count and multiply the regime length by 3 and you get Fidel Castro.
The Chilean armed forces (navy, army, air force, police) intervened when they saw an imminent civil war, combined with a likely Argentinean invasion.
That would be effectively the end of Chile.
https://en.m.wikipedia.org/wiki/Beagle_conflict
https://en.m.wikipedia.org/wiki/Operation_Soberanía
And Allende's regime was very far from suceeding. He had inspiring speeches but in practice he was a failure as a leader. Allende was a demagogue on the KGB's payroll with drinking issues, his administration was undistinguishable from anarchy.