Then why serbs didn't use C-75 to do the job?

They not had any operational SA-2 during OAF...

Between 1962-66 Yugoslavia fielded 4pcs SA-75MK Dvina (SA-2B/F) around Belgrade. (250th SAM regiment)
Between 1966-67, 4pcs S-75M Volhov (SA-2E) around Zagreb. (155th SAM regiment)
During 1974, 4pcs S-125M Neva (SA-3B) around Ljubljana. (350th SAM regiment)
During 1977, 4pcs S-125M Neva (SA-3B) around Skopje. (450th SAM regiment)
In the beginning of the 80's 4pcs S-125M Neva (SA-3B) was added to the 250th SAM regiment, increasing its strength to a brigade.

When the breakup of Yugoslavia happened the 350th regiment was disbanded, and its 4pcs Neva was strengthened the 250th around Belgrade, while its Dvina's were decommissioned.
The 155th SAM regiment was sent to Bosnia, and done nasty things using "Zemlya" against cities.
The 450th regiment was redrawn around Kraljevo.

According to the Dayton Peace agreement, the 155th regiment was disbanded, and its Volhov's were scrapped.

Thus in 1999, eve of the Operation Allied Force, the Yugoslavian Air Defense had:
Three radio technical Brigade (20., 31., 58.), with 17 radio technical battalion.
Two fighter regiment (204., 83.) with 14 MiG-29, 2 MiG-29UB, and 44 MiG-21bis fighter.
One S-125M Neva (SA-3B) brigade (250. around Belgrade) with 8 systems.
One S-125M Neva (SA-3B) regiment (450. around Kraljevo) with 4 systems.
Five 2K12M KUB (SA-6B) regiments altogether with 20 systems.
60. sr PVO around Podgorica
230. sr PVO around Nis
240. sr PVO around Novi Sad
310. sr PVO around Kraguljevac
311. sr PVO around Pristina

During the war, the Serbs tried to bring one decommissioned SA-75MK Dvina (SA-2F) system back to life, but NATO prevented it by bombing the system during its adjustment.