I would posit that Hughes was writing for a more mature audience than Longfellow was. Whether that is delineated by age, by experience, or whatever, these two verses are distinct in their depth, even though both are similarly brief. In all fairness there are two more stanzas to the second poem, while the first is self-contained. But IMO this is a valid point to make - one wants to spend more time musing about the first. Personally I enjoy the efficiency of words - a lot is said in only a few. That’s difficult to do.
Can we analogize this example to one of, say, mathematics? A simple yet sophisticated theorem is best understood if one has a mathematical foundation in the first place. That will require going through elementary arithmetic before moving on to algebra, geometry, calculus, etc. Is Hughes’ work best understood and appreciated if one has some experience reading some poetry in the first place? Children tend to move through literature the way they move through mathematics - starting simply, but getting more complicated with age, experience and time. One sort of builds up a literary vocabulary and set of thinking skills the way one would build up the same for mathematics. Or is it more about enjoyment of the poetic work and forget taking the closer look? There are analogies to math in that framework, too. To enjoy either literature or math seems to require some inherent attraction to that way of thinking.