That would be because when you do a reset of a phone that has an SD card that has been converted to "Internal" storage, which it would have to be to run apps from it, that card is encoded. Once you reset that makes the phone unable to access the data on the card. Just like if you had put the card in a different device. The data becomes inaccessible by anything. Basically the data is there, but no way to access it. This is one of the drawbacks of making the SD card "Internal" rather than "External" storage. There isn't a way for you to get the data back.