I would say that some poets don't have many readers because of the sorts of poems they write, but that the reason most poets don't have many readers is because of the ways that poetry gets taught in this country--or not taught, as the case may be--from grade school onward. If poetry were taught, for example, as part of the national patrimony, with the assumption that elementary students should know their Whitman and Dickinson, Hayden and Hughes, because these are our great national poets, just as French students and Italian students learn Baudelaire or Petrarch early on, there would probably end up a wider audience for poetry.
I'm continually surprised by how thoroughly the idea that literature is ulimately something to be taught in schools has become unexamined wisdom. At best, works of poetry or fiction might be useful in building vocabulary or learning the role of figurative language, or as supplements to other courses such as history or social studies, but in these instances literature itself is not really the "subject" at hand. (Which itself is problematic. If literature is finally pedagogically helpful only as something that can be added for illustrative purposes to other areas of study, most students are likely to view it in their future lives as nonessential, purely as the means to another, more important, end.) To study poetry as poetry requires a sensitivity to language and to the possibilities and purposes of aesthetic form that, frankly, most teachers lack and most students have no interest in developing. But I'm not sure why either should be expected to possess such sensitivity. A taste for poetry is a minority taste, has always been a minority taste, and, despite all the efforts to make poetry "relevant" to readers who otherwise couldn't care less (perhaps because of them), will always be a minority taste. And I don't understand why this is considered to be a problem. Physics is a minority taste as well, but physicists don't seem to spend much time brooding over the fact that most people don't read physics journals..
Thus the only really feasible argument on behalf of teaching poetry in school is the one Selinger advances here--"as part of the national patrimony, with the assumption that elementary students should know their Whitman and Dickinson, Hayden and Hughes, because these are our great national poets." But this "national greatness" approach is probably the very worst way of convincing young readers to give a poetry a chance. Whitman and Dickinson get elevated (or, more accurately, reduced) to the level of all the other "great" historic figures whose achievements are duly recited but otherwise ignored. To tell these young readers that such poets are part of their "patrimony" is only another way of saying they should read poetry because it's good for them, a strategy that will only make it certain they'll never want to read a line of verse ever again.
I favor the opposite strategy: take literature out of the schools altogther, even out of college (at least as a required course). Those who might realistically develop an interest in poetry or serious fiction will come to it not because it's been forced on them but because it's been freed of all taints of the "academic," of something one learns in school and then forgets. Emphasize the way poetry embodies values and practices that are precisely at odds with the conformist attitudes enforced by "school." Take it out of the classroom and let it breathe.