#3 He's a part-timer

A number of Superstars down the years have worked a part-time schedule, but hardly any of them held WWE's top title. A part time athlete should primarily work as an enhancement talent for other talents and not to constantly dominate those he comes up against.
In a period where WWE needs to produce talent, having Lesnar go around crushing all his opponents makes little sense, especially when it is on a part-time basis.
Details of Goldberg's last match HERE
With a potential UFC match rumored to be in the pipeline, The Beast will have to devote most of his time to train for the bout. To have the Universal Title on him at such a time isn't the most sensible decision.
Whilst Lesnar is clearly a valuable asset for the WWE, he won't be with the company in the long run and therefore should not be a long term WWE Champion. Lesnar is already established and could be better used working in non-title singles matches.