Why?

As Bob Shaw observed in a Serious Scientific Talk at a Science Fiction Convention, most people think that, if they are struck by lightning, it will kill them but science fiction readers know that a much more likely outcome is that they will be flung into the past.

Which period of the past they will be flung into is a product of three factors:

the voltage of electricity in the lightning;
their body weight, measured in pounds;
which period the author has been mugging up on.

Lightning was the mechanism for time travel in L Sprague de Camp's Lest Darkness Fall and in Poul Anderson's "The Man Who Came Early," which is a direct comment on the de Camp novel. The Anderson story remarks that lightning-induced time travel happens very rarely - so rarely that it has happened in only one other work of fiction so far?

A similarly humorous explanation may be appropriate for the conundrum presented in Anderson's "Delenda Est":

"'...the Patrol and the Danellians are wiped out. (Don't ask me why they weren't "always" wiped out; why this is the first time we came back from the far past to find a changed future. I don't understand the mutable-time paradoxes. We just did, that's all.)'"
- Time Patrol (New York, 2006), p. 187.

I suggested two answers to this not-to-be-asked question in a recent post but the real answer, of course, is that the author has started to write a series about an organization whose job is to prevent time travelers from changing the past so now he is writing a story in which some time travelers have changed the past. (Similarly, Asimov started a series in which Seldon's psychohistorical predictions guided the Foundation, then wrote a story in which the Mule overturned those predictions - but, as I always say, Anderson's historical fiction is far better and moreover is about real history.)

When the Patrol plans its counter-intervention in the pivotal battle, Everard says:

"'...I don't think we can get away with more than two agents actually on the scene. The baddies are going to be alert, you know, looking for counterinterference.'" (p. 222)

This tells us that the action will continue to focus on Everard and his companion for this story, Van Sarawak. Indeed, Everard continues:

"'The Alexandria office can supply Van and me with costumes.'" (ibid.)

Something else that we might notice here is that, as science fiction readers, we are on familiar territory. We already know that, if we intervene in a battle in order to change the course of history, then we need to be alert for any other time travelers trying either to preserve the established course of history or to undo our changes!

The tradition of time travelers possibly observing and interacting with historical battles goes back to a conversation between the Time Traveler's dinner guests.