Men on paying sites tend to be a lot more serious about meeting a romantic partner and being in a serious relationship than men who use free dating sites.
Being that they have paid their hard-earned money to join a dating site, these men are going to be a lot more invested in finding someone.
They are most likely already chatting with several people on the online dating site.
When they drop off in mid-conversation, they may have decided to focus their attention on someone else that they are connecting with on a deeper level.
It may seem odd that a person would just drop off in mid-conversation with you, especially when they seemed to be so interested in you, but this isn’t an uncommon occurrence on online dating sites.