Publicity stunt mostly. Plus the general heavy handedness of the BATF. IOW, pretty much their normal SOP, only more so.
The reason the Davidians knew they were coming was that they had notified the local media and one of them got lost. The person they asked directions of was a Davidian, who promptly notified Koresh that the BATF was coming. The BATF under cover agent was at the "compound" when Koresh received this message, and reported back to his superiors that surprise was lost. The dumb SOBs went ahead with the assault, which depended on surprise for any chance of success, anyway. And had their a$$es handed to them by the Davidians.
They were MURDERED for a Klinton publicity stunt? There's gotta be more than that. Koresh knew something, and the Beast/Whore didn't want anyone to ever know.
One thing that was finally admitted by the government is that there were NO automatic firearms found and later that none were really suspected.