We always appreciate peace and quiet the most when we've just escaped from a lot of noise and clatter. The world of "Tintern Abbey" is no different. The speaker has done his time in cities and towns, and he missed the heck out of the country while he was there. While he was in the city, he was sustained only by his memories of the beauty of nature. In actual fact, Wordsworth did spend a year or so in London, and wrote about the experience in his semi-autobiographical poem, The Prelude. Suffice to say, London was not his favorite place.
The speaker of "Tintern Abbey" is only able to develop his consciousness of the "presence" in Nature because he spent some of the intervening years in "towns and cities" (26).
In "Tintern Abbey," the speaker's development is actually hindered by the time spent in "towns and cities" (26).