Good way to cover Goldbergs ring rust... The WWE using it as a way to punish Brock for using banned substances and making him lose his aura of being a beast... Building them both up to enter the rumble and have a shot at the WWE title without making Brock seem too OP. Maybe none of those reasons.
It's actually a combination of a lot of different things going by all the recent reports.
To sum up.....
Brock was originally scheduled to win, but after all the positive reaction for Goldberg's return, and Lesnar saw money in the idea and even pushed for a longer programme.
Goldberg being injured on the RAW with the security guards was another rumoured factor to count in, but it ended up not being true.
The bookers/creative actually wanted to get the dominant and squash heavy "WCW Goldberg" back, not his watered down WWE 2003-2004 version. History has shown that long matches for Goldberg is his biggest weakness.
Given this is the first time Goldberg's wife and kid gets to see him wrestle, he has his WCW theme music back, playing the Superhero for the kids of this generation, and then the idea of Lesnar underestimating the myth and legend of Goldberg aka the monster long before The Beast himself, it kind of adds up.
I'm sure there are plentiful of people who are tired of seeing Lesnar phoning it in ever since Suplex City was his thing, but for all intents and purposes, he's actually a damn good seller and if this squash match was any indication (Seriously look at the way he was staggering and held his ribs), he can put the effort if the money and entertainment is there.
Goldberg destroyed tons of big names of similar fashion in the past, something people seem to forget or never knew, so this was the return of another attraction. It doesn't detract from everything Lesnar has had since.
Both are also great friends in real life, don't forget that.