Simply because... with a long credit history, a prospective lender can see how you have operated your credit facility over a considerable period. Nobody's credit history is perfect - everyone has at least 1 or 2 lapses on their record. Having just a short credit history doesn't show 'trends' or patterns of (perhaps) persistent late payments.
A long history with corrected blemishes exhibits that though mistakes were made, they were corrected which allows for those viewing your credit history to know that you've learned to fix mistakes making you trustworthy and experienced. Meanwhile, a short clear history doesn't provide enough information and dose't show what could happen overtime and how you would handle it.
Explanation:
Whether you’ve had credit for six months or 20 years can make a big difference in your credit score. A long track record without any major slip-ups suggests that your credit behavior will be similar in the future — and lenders and credit card issuers like that. Being an authorized user on an old, established account in which the primary cardholder has excellent credit may help your score a little, but the passage of time during which you build or maintain good credit helps the most. Keep the length of credit history in perspective: It’s only one element influencing your credit score, and not the most important one at that.
