Extracted from a rather good profile of Peter Sis:

Sis admits that his approach to storytelling — described by some as “cerebral” — has been a strength as well as a deficit, especially in the face of editors who weren’t sure that his sensibility was right for kids. He came over originally as an animator, and found this reaction to be a continuity between the two fields.
“I started to shop my own ideas, and very often I would be told that it’s too cerebral and it’s not American and lots of people told me to go back to Belgium,” he said. “Then the same thing started to happen in the books. They said your ideas are way too serious, too cerebral.”
The un-American quality of Sis’ work became a reason for some editors to attempt micro-managing, to the point where they were directing him to draw bigger eyes on faces, so his characters didn’t look as foreign. Eventually, Sis was able to adapt ordinary American aspects to his stories in a more natural way.

via Peter Sis: Accidental children’s book legend – North Adams Transcript.

Sis adds at the end of this piece:

“All those houses that I used to know 25 years ago, now it’s down to three big corporations, which are merging and merging. It used to be seven different publishing houses, which had their own identity. In that sense it’s very difficult. Illustrators will be dealing with basically three art directors, who will have to decide if this fits the mainstream market.
“Maybe it’s because I’ve been around the block too long. Could be that when we get older, we get more skeptical. Maybe there will be some other new ways how to do it, but I don’t know at the moment. I’m in this situation where I feel a lot like Maurice Sendak, that there is no publishing left, there are only three editors.”