Because heavy sets are still good due to their high p.def and hp(doom heavy, fp++), or sometimes due to other boosts like +7 speed in the case of bw heavy. But suffer more against autoatacks (like from tanks, bards, summons, archers) cause have less evasion.and no critical chance reduction from autoattacks like light armor passive. You ll begin feeling the effects in high lvl with all skills learned. Until then, pl best for pve +12 str -15 con, +bow like pose said. if want also to pvp bw heavy ll do the job.
Do not forget that in the next patch bw sets ll give stun resistance and doom sets root and sleep resistance!
Before you get high level i guess admins ll reopen test server here for zaken so no need to check in other servers. Gl