That's why they caved.
Tactically, the Bundys and their supporters surprised the Federal Officers. They did not expect the protestors, particularly all of those well armed militia protestors, to come to them.
When they did, the protestors did some very key things, and were probably doing so as a result of their militia support,, many of who are former military.
Notice on the highway that the supporters placed fire support on the bridge with over watch positions on the Federal holding pen.
This was on the interstate highway. And there were several such shooters up there.
Notice how almost all of the Federal armed officers, who were also heavily armed, were behind several BLM SUVs near the gate:
That position was being flanked on both sides by armed protestors, was being approached directly by a large group of armed protestors to their front.
The position of the federal officers was untenable. They had their own binoculars and they knew what was happening. I also believe most of them did not want to be involved with a fire fight with American citizens over a bunch of cattle and tortoises.
What prevented a slaughter was the commitment and determination of the Bundys and their supporters, and the fact that they were as well armed as the Federal officers and significantly more numerous.
As with almost all situations, being strongly and well armed is a tactic that prevents violence...as it did in this case.