Why do many critics of children's literature say that Eloise in Paris is less successful than the original, Eloise?
Sequels are rarely as successful as the original.
No matter how you define “success,” it is not uncommon for a sequel to be less successful than the original. Often the original will make a splash, and later installments will be popular because the first entry was. This is especially true with classics like Eloise.
One way that success is measured is in numbers. In this case, it would be how many books are sold. For example, on amazon.com, Elosie in Paris had a rank of 47,749 in Books. The original Eloise has a rank of 19,734. That is actually quite a big difference, and Eloise does seem much more popular than Eloise in Paris.
You mentioned critics. Critics will usually compare books based on other factors besides sales rank. However the professional review I found was positive for Eloise in Paris, but glowing for Eloise.
Here is an excerpt from Eloise in Paris:
Once again Eloise sets about turning the world around her upside down. Her precocious little voice practically leaps off the pages complete with many "me ELOISE" and lots of exclamations, conversations, and narratives.
It’s a positive review.
Compare that to this excerpt from The Absolutely Essential Eloise:
There can be no doubt that Eloise is one of the most fascinating, entertaining, and delightful picture book characters ever created, and this first book about her is a classic.
While the first review was positive, this second review is about as positive as possible.