fiction - Why does word-count matter for a debut author?

Background

I'm not sure if this is a bit of a self-explanatory question to many people, but I just can't figure this out. Tons of blogs made for writers, magazines, and other forms of media state that a debut author should not try submitting a manuscript of more than 100,000 words (or about 110,000 for fantasy). I don't think I've never seen anyone deny this.

Question

So,

Why do publishers not want to publish lengthy work from debut authors?

Personally, I think the story I'm writing would work best as a long, single book, about 150,000 words in total. I feel like if I were to start a second book in the series, I'd have too little to fill it with. My work would also work better if it were self-contained in a single book.

Would I be able to get away with what I just described, as a debut author who has never been published?

Thanks.

Answer

I think I read this in the Sell Your Novel Toolkit, but I may have seen it elsewhere:

Retail stores have limited space. Debut authors (except for a very limited few) have very limited shelf space. You might get 3 books on a shelf (300k words of paper). But you're only selling 2 books with 150k words, not 3. So all of a sudden the risk just went up for the retailer and publisher by quite a bit.

One might argue that the readers themselves aren't willing to give a longer book a fair chance, but I'm unsure how true that is.