I have been on my server since 2005 when it was first launched and in that time my server has gone from being a medium population server (ever so briefly) to being a low population server less than a year after going live. Since then, it has remained a low pop server. While at the time it wasn’t a big deal, the population on the server has steadily declined. During Vanilla, before paid transfers were implemented, most people rolled on a server and stayed on that server until they either stopped playing or found a reason to start over on a different realm. If you reached level 60 on a character, people tended to stay on that server because it was such a pain to re-level on a new server. Once paid transfers were introduced the barrier to leaving your server was removed. Players were free to transfer their characters to new servers to play with friends/coworkers/raiding guilds/etc.
Of course this didn’t affect the larger realms since the higher population numbers meant they could afford to lose some people to other servers without it affecting the overall population. Unlike larger servers, the smaller servers suffered because the realms couldn’t afford to lose so many people. As new people began playing WoW, most would pick the larger realms because the population made them more appealing than the low pop counterparts. Who wants to roll a new character on what appears to be an empty server when they can roll on a busy server that looks to be more fun? Not all transfers were off of low pop realms. Some people transferred to smaller servers to play with friends or because they grew tired of life on a larger server.The transfers in weren’t numerous enough to slow the tide of people leaving.