How do you account for the differences in subject matter among Bryant, Holmes, Lowell, Longfellow, and Whittier?

There is no doubt that people are often the products of their age, and poets such as those mentioned certainly exhibit characteristics of the literary movement in which they lived or about which they have read. For this reason, there is a difference among the poets cited in their subject matter.

• Romantic poets delighted in nature's beauty and its solace and communion with man.

While poetry is often an occupation in which one engages alone, it is not always lonely. Poets rarely compose their verse in isolation from the influence of other poets of their time or their predecessors whom they have read and loved. For instance, when William Cullen Bryant was young, he read a book of poems entitled Lyrical Ballads by William Wordsworth and Samuel Taylor Coleridge, two of his favorite British contemporaries. This volume of poems reflects the philosophy of the Romantic era, and it inspired poets who wished to replace the conventional poetic diction with common speech. Bryant felt himself akin to them and carried the messages of British Romanticism into his poetry. His romantic poem "Thanatopsis" certainly reflects this delight in nature and the lessons that it teaches man. Rather than being a dirge, in Bryant's poem about death, the verses exalt the body's rebirth into the earth and consolation from Nature.

To him who in the love of Nature holds
Communion with her visible forms, she speaks
A various language.

Another Romantic poet is Henry Wadsworth Longfellow, who also writes about the interrelationship of man and Nature as exemplified in the last rhythmic stanza of "The Tide Rises, the Tide Falls":

The day returns, but nevermore
Returns the traveller to the shore,
And the tide rises, the tide falls.

• Fireside Poets - These poets delivered messages of morality and sentimentality and employed very conventional forms. Their poetry affected the era and was quite popular until the arrival of the 20th century, when conventionality and sentimentality lost popularity.

Two Fireside Poets are John Greenleaf Whittier, and Oliver Wendell Holmes. Whittier wrote his poem "Snow-Bound: A Winter Idyll" in a natural, conversational style, a style that also suggests a romantic interlude:

The wind blew east, we heard the roar 
Of Ocean on his wintry shore,
And felt the strong pulse throbbing there
Beat with low rhythm our inland air.

In addition, in his poem "The Chambered Nautilus," Holmes contemplates nature and is in tune with the Transcendentalists, Emerson and Thoreau. This poem seems a transitional poem that suggests the more exuberant movement of Transcendentalist thought, as exemplified in the last stanza:

Build thee more stately mansions, O my soul, 
As the swift season roll!
Leave thy low-vaulted past!...
Till thou at length art free
Leaving thine outgrown shell by life's unresting sea!

Oliver Wendell Holmes's poem "Old Ironsides" uses vivid imagery as well as verbal irony to praise the USS Constitution and to protest the proposed scrapping of such a historical ship. He also uses a forceful meter to convey emotion in the protests of the ship's destruction. 
This poem and others by the Fireside poets, while popular for many generations, now do not appeal as much to students, as they seem rather stilted in their conventional constructions and sentimental themes.