Question: Background
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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
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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.
The reason why is economics. 

Why do I, as a publisher, want to take a risk on an an unknown author who has written a 150,000 word book? That's money we'll be spending without guarantee of a return, and less books we'll be able to stack on a bookshelf. 

I can take that risk if your work seems absolutely brilliant and destined for greatness, but that's not an easy thing to predict. Economically speaking, best to hedge my bets by taking shorter works for new authors and going big only with authors you know will give you a good return-on-investment.