When choosing a form for a poem, poets are most likely to consider the impression it will leave on readers. The form can significantly influence how the poem is perceived, how emotions are conveyed, and how effectively the poet communicates their message. Poets think about rhythm, rhyme, line breaks, and overall structure to enhance the emotional impact and clarity of their themes.
While the seriousness of the topic can also inform their choice, it's the impression—how the form contributes to the reader's experience—that is often a primary concern. Other factors, such as the ease of using certain structures or how long readers will spend on each line, may be considerations, but they tend to be secondary to the overall impact and effectiveness of the poem.