I think becuase it gives readers of the f/s a truer picture of the company.  Basically, the case here is - a) separate line item/disclosure or B) buried within R/E.   

The losses from development stage could cause the overall retained earnings to be negative.  Perhaps the company is out of development stage now and is turning a profit.  It would be useful to show a separate line item that the negative R/E is due to development stage, but the company is now profitable.

Just common sense really - if in doubt, more disclosure is better than less.  The key to CPA is not memorizing everything.   I don't think it's a requirement to know everything...what they want you to demonstrate is having the framework down to know what GAAP and all that jazz is trying to achieve.  I can't explain myself well here, but you need to approach the questions as a user of F/S's...if you can keep this frame of mind then you can answer alot of questions correctly even if you never came across the material before.  And that's why so many people fail.  You need an accountant's mind...not just doing arithmetic and garbage like that.  Once you see the light, the exams are a piece of cake.