Because despite bug history of OpenSSL, nothing proved to be more reliable to this moment.
Huh? Of the 22 vulnerabilities OpenSSL has disclosed since March (4 high severity, 14 moderate, 4 low), LibreSSL has been vulnerable to 8 (0 high, 6 moderate, 2 low).
References:
March: https://marc.info/?l=openbsd-cvs&m=142677372515025&w=2
June: https://marc.info/?l=openbsd-announce&m=143406498020131&w=2
Today: https://marc.info/?l=openbsd-tech&m=143645910727507&w=2