He went on tour. That's it, it's pretty simple. You can't have a voice like Axl's and not have continuous shows impact it. Vocals get worn out, also there were probably shows in 87' that he forced it a little too much or his vocals weren't up to it but he performed anyway - stuff like that just adds to vocals being damaged and/or strained. That "tender" sound you're talking about is because it hadn't been put through the ringer yet with the appetite tour. No doubt he was discovering new ways to sing and express his range and perhaps he did alter his style somewhat to make it through tours but going through 87' to 88' had an impact on it.