Why did Wordsworth write about nature in most of his poems?

Just to elaborate upon the answer given, you can just have a look at the different ways in which Wordsworth and his contemporary romantics were portraying nature. The philosophical sources are diverse. There is a good deal of German transcendentalist connection here with influences of Kant and Schiller. Rosseau's dictum "Return to Nature" is yet another impetus. There is a Spinozian element too. Let us look at some of the ideas Wordsworth and fellow Romantics were trying to communicate through nature--

1. Nature as in purity, innocence and man's relation to nature at the level of sensations...his growing experience as in Tintern Abbey

2. Nature in its inward form--human nature--the affective space within. All this was a reaction against the Neo-Classical fixity with urbanity.

3. The politics of nature...Man's manhandling of nature, the movement away from nature critiqued as a metro-centric neglect of rural life in all its simplicity.

4. Nature as a way of approaching God...the mystical experience of it...Spinoza's pantheistic notions of Natura Naturata and Natura Naturans come in over here..e.g. The Prelude

5. Nature as an impetus/stimulus for poetic imagination...