You may have been in this spot: a position has come open within your company and you wrote your resume and submitted it. You were one of the finalists for the position but didn't get the job so you decided to look around outside the company to see what the market offers. Your resume worked great for the internal position so you assume it will do the same for you outside your company. You send it out and nothing happens-- no calls, no emails, nothing.
The reason an internal resume usually does not work outside the company is simple. The internal reader has a frame of reference into which he/she can place the content of your resume. If you mention a project on an internal resume, you generally don't give details because the reader is already in the company and knows what that project entailed. The reader knows the impact that project had on the overall company mission or where it fell in the scheme of things for the company. You knew this frame of reference was there before you prepared your resume so you subconsciously did not go into detail.
Another advantage to an internal position is that you have a reputation established within the company. If you don't know the hiring manager directly, chances are he knows someone who knows you. The hiring manager might talk to his internal connections to check you out and also talk to your supervisor to get the skinny on your performance. The grapevine kicks in to supplement your internal resume.
When you field the resume to an outside position, you don't have either of these advantages - point of reference or internal recommendations - so you have to overcome that as much as possible with the resume. The resume is your primary marketing tool for external positions and it needs to sell your skills better than a resume for an internal position would. It has to do a better job because the hurdles are higher on the outside. Most people don't understand this concept so they go after jobs on the open market with a resume that might have been prepared for an internal position.