Why? Basic insurgency dynamics. The insurgent force has inferior strength, therefore must rely upon surprise and striking soft targets. DC is right now the precise opposite of that.
Violent attacks are more likely at other targets that are not being so heavily defended, or even at targets that are not being much defended at all. It is simply not possible to defend every target of actual or symbolic value.
For example, virtually every community, even many very small ones, has a post office. Any community of appreciable size has multiple post offices. Such facilities are symbolic of the Federal government because the Postal Service is run by the Federal government. There is no way that every post office in the country can be secured.
And that is just one class of symbolic targets. There are Forest Service offices, BLM offices, National Wildlife Refuges, National Parks (and Monuments), FEMA facilities, Federal office buildings, etc. scattered all over the map. (Military facilities, too, but those are not soft targets and thus unlikely to be attacked.)
The only possible exception to the unlikelihood of an attack in DC, one that cannot be ruled out, is an inside job of some sort, one involving disloyal Federal forces. Measures are being taken against such possibilities, but they might not be successful. I will still go out on a limb and say such a thing is not likely.
Somewhere, somehow, something probably will be attacked. But a direct attack on the inauguration in DC is unlikely. If one happens, it will prove to have been a suicide mission for the attackers.