It's still early in the morning when Wordsworth composes his famous lines, so naturally there aren't many people about. And even if there were, it's unlikely he'd mentioned them anyway. Because in the sleepy city, Wordsworth sees something similar to the joys of nature he normally writes about. It's as if he's transposed the natural world, with its living, beating heart, to an urban environment.
The city is not really a congenial place for Wordsworth or his poetry, so the only way he can write about it is if he can treat it like an extension of the countryside. Among other things, this means capturing a moment in the city's life when there's no one around. It's the beating heart of the city that interests Wordsworth here, not the people who live in it.
Wordsworth does not speak of the peopled bustling city because his subject is the comparison of the city’s calm in the early morning (before people are out of bed) to the calm of nature: "Earth has not anything to show more fair...." He is struck by the way the city’s structures (“domes, theatres, and temples”) are like Nature’s own creations--“valley, rock, or hill”--and how the calm river Thames seems to have a will of its own. In this fourteen-line personification of the city (“that mighty heart”), Wordsworth is tying humanity’s existence to Nature, a primary Romantic observation.
In other poems Wordsworth does treat the city population in considerable detail (see The Prelude and "London, 1802"); for example, in The Prelude, "Residence in London" (639-649), the blind beggar on the street seems to admonish him as if "from another world”: "I gazed, / As if admonished from another world." Note: In analyzing poetry it is always dangerous to inquire into what is not dealt with. Remember Eliot's admonishment: "A poem must not mean, but be."