  The extragalactic $\gamma$-ray sky is dominated by blazars and their study
plays an important role in understanding jet physics, cosmic evolution history
and origin of ultra high energy cosmic rays. In this work, we study a large
sample of BL Lac objects to investigate why some sources are detected in
$\gamma$-rays, while others not. We selected 170 BL Lac objects, with measured
synchrotron spectral curvature and Doppler factor, and divided them into
Fermi-LAT detected (FBLs) and non-detected (NFBLs) sources. We show that FBLs
have smaller curvature than NFBLs, even after getting rid of Doppler beaming
effect. The BL Lac objects PKS 0048-09 and S5 0716+714 have similar synchrotron
peak frequency and luminosity but different $\gamma$-ray dominance and their
quasi-simultaneous broadband spectral energy distributions (SEDs) can be well
fitted by a log-parabolic synchrotron self-Compton (SSC) model with same jet
parameters except for the curvature and source size, assuming curvature being
proportional to the size of emission region. Our results imply that for a given
synchrotron luminosity, the different SED curvature and Compton dominance may
account for the discrepancy between FBLs and NFBLs. We discuss these results in
context of stochastic particle acceleration and radiation mechanisms.
