This is an excellent question, and I'd like to direct you to the Library at UC SanDiego. They have an extensive Seuss archive in their Giesel Library (https://library.ucsd.edu/research-and-collections/collections/special-collections-and-archives/collections/the-dr-seuss-collection.html), and if you send your inquiry using their "Ask Us" feature you will likely find an expert! Another resource you could try is the Seuss museum in Springfield, MA, who might be able to direct you to other Seuss archival information.
There are a number of reasons why a finished manuscript might have taken a long time to publish. It could be that when these books were first written, Seuss's agent shopped them around to different editors at publishing houses and there were no takers, so the projects were shelved (this happens to many writers, who might sell a later project first and then recieve offers on their older manuscripts, but seems unlikely for a writer of Seuss's renown, who published consistently with Random House during this period of his career).
It is also possible that there was an aspect of the script or the illustrations that Seuss was unhappy with, and so he held off on selling the nearly-completed books. After his death, the inheritor/manager of his estate would be responsible for deciding to pursue publication or keeping the manuscripts in an archive.
And the book making process is very slow, even after the book is done! A book needs a contract, which can require months of back and forth negotiations with an agent, especially on a project as important as a Dr. Seuss book that everyone knows will make a lot of money. An agent would be in an excellent position to negotiate a standard, boilerplate contract to something more advantageous, like a royalty rate that increases after a few thousand copies or receiving an advance in full prior to publication. Or, a publisher might decide to delay a publication date so that a book is published in a season where it will sell well (a Christmas book released in the late fall season, a story featuring children released in late summer to target people looking for back to school gifts, etc.). The publisher might wait for subsidiary rights to sell (like the right to make merchandise, or produce a tv show) so that the book can be marketed along with all of its licensed products. There can be delays in production (maybe the person who needs to sign off on the proofs is out on leave), or a printer could be backed up with orders.
There are so many factors involved, and the reason that these specific titles were published later could be for any of the above reasons or a different one entirely.
Best of luck tracking down an answer from the Seuss resources!
