Hi
The reason that you are not cleared AT a specified level is for the fact that you are VFR and so therefore maywell mot be able to comply with that instruction all the time. Therefore it is easier to say 'GABCD cleared to transit the zone not above alt xxxx QNH' That means you can fly whatever level you want up to that level and subject to the usual legal restrictions.
The reason most probably is to protect IFR departures or arrivals above you.
Maybe a radar guy can give you a better more comprehensive answer.
Hope that helps
LORI