Nintendo UK’s Ben Taylor talked with VG247.com and was asked about the lack of the second circle pad on the Nintendo 3DS XL and here is partly what he said in response:

“We did review the possibility of the second controller, but we felt that increased battery life was really important to players, and this would have been affected if we had introduced one. It would also have made the console a lot bigger and bulkier.”

Taylor here brings out that Nintendo did think of the option of adding a second circle pad to the Nintendo 3DS XL, but decided to swap that add-on for increased battery life. Having the second circle pad would of hampered the the battery life and Nintendo decided that it was more important.

Source: VG247.com