But why is it ‘harder’ to sell a book if you submit both text and illustrations? Why isn’t it more of a ‘can’t hurt, might help’ sort of scenario?
It's a common misconception that editors/agents 'need to see' the pictures to 'get' the story. Actually, good editors can bring a lot of imagination to what's possible based on their years of exposure and professional analysis of audience,themes, trends, and market. Imagining the 'right' illustrator plays a role in this. I believe one of the reasons I was able to sell my first children's book was, in part, that the editor imagined it in the hands of a certain up and coming illustrator, and it 'fit' perfectly. Why should I art block that?
You want text finalized with the editor before you do much illustration work (another reason editors prefer to see the text first). Sending illustrations with text that may require some work (removing a redundant character, extra verbiage, etc) makes the whole project seem like a huge, unnecessary headache.
Sendak had his story submitted before he started illustrating. His original title was "Land of the Wild Horses" until he realized he couldn't draw horses. It was his editor that suggested changing horses to 'wild things'.
(Speaking of which, If you really want to get into the head of a children's book editor and what it looks like to work with authors, illustrators and author/illustrators, read the delightful and insightful Dear Genius, The Letters of Ursula Nordstrom.)
That's why experience counts. Geisel (Dr. Seuss) spent 10 years establishing himself as a cartoonist and illustrator before he wrote and illustrated his first book. Maurice Sendak was best known for illustrating Else Minarik's Little Bear series long before he wrote and illustrated Where the Wild Things Are.
Still, neither of these fellows breezed into publication.
So yes, IN GENERAL, you're going to have a greater chance of selling your book with text only, or getting a gig for a different book with illustration samples only, because submitting text and illustrations as an entwined unit greatly increases the chance of imperfections here or there being perceived as all wrong.
As an aside: as a writer, I would never, ever, hire an illustrator to illustrate my text and then submit it to a children's book publisher (unless it's a self-publish, in which case you're the publisher and that's a different story). I can't begin to tell you how may things are wrong with that.