Because Buk-M1 and Buk-M1-2 TELARs' acquisition radar
covers only 6-7° elevation (and 120° bearing). After the TELAR crew was notified "bird flew to you", they turned on the acquisition radar, the first aircraft which entered the narrow scanned area was locked - the radar switched to target tracking, then target illumination (in these modes the radar covers only 2.6° elevation and 1.4° bearing).