Why ”were” and not “was” in “and e’en to tell it ᴡᴇʀᴇ no easy task”?

This is an archaic use of the subjunctive. It may have been somewhat archaic even 200 years ago. It means

to tell it would be no easy task.

If he had written

to tell it was no easy task,

it would mean that he had already told it (which he hasn't; he's explaining it's savage and wild beyond his powers of description).

And if he had written

Gone from the path direct; and e’en to tell​
It would be no easy task, how savage wild​

the line wouldn't scan properly.

From a different translation, which I think is somewhat more literal in this passage:

Oh what it was is a hard thing to say,
so overgrown with things savage, harsh and raw.
The thought renews the fear in me today!