Because a pilgrim is a traveller who is always on the way to something he has not yet reached, carrying almost nothing and depending on what the road provides — and that is exactly the condition of the human heart in this poem.
Take the three things the word carries, and match each to the extract:
- A pilgrim is journeying, never settled. “upon earth” makes the whole world only a road to be crossed. The heart is not at home here; it is passing through, still looking.
- A pilgrim is in need. The very next line says so — “Finds often, when it needs”. A pilgrim on the road asks for water and shelter; the heart asks for comfort, understanding, truth.
- A pilgrim seeks something sacred. That raises what the heart wants above mere pleasure. It wants something worth the journey — and it is handed “just so many weeds”.