Private servers are free, but it comes at a priceAren't private servers free? Why would people pay for what they were already doing, with a team of people that actually listen to them?
1. servers usually aren't very stable. DCs and rollbacks happen every now and then
2. almost always based somewhere in Europe, so ping is ASS
3. servers deletions happen, so you can lose your hard earned characters at any point
4. a lot have p2w aspects (eg warmane sells pretty much everything from gear to instant max levels)
5. there's usually big competition between private servers, and it's not uncommon for a massive section of your server population to simply leave forever because fagbois™ new and improved server was just released, and everyone wants to go play there now
That's all I can think of off the top of my head. Official servers should solve all of that