[one or more factors that make one popular etc.]だからね
This is a common phrase to acknowledge someone for being pretty good in some way (popular, smart, etc.) either in front of them or when gossiping about them.
It's basically "It makes sense how he's popular with women when you know that he's single and handsome."
Japanese people fill in that sentence like this in their minds:
- そりゃ slang for "of course," "naturally," "then"
- モテる "to be popular with the opposite sex"
In the sentence you wrote, it sounds like the speaker is talking to their friends about how some guy they know is popular.