Easy enough. RP-PvP servers…
When World of Warcraft first launched, there were no such things as RP-PvP servers. Plenty of “dead” servers, as you call them, and plenty of RP, PvP, and PvE servers. But there weren’t any RP-PvP servers at launch. All RP servers were PvE.
But after enough people asked for them, Blizzard introduced the RP-PvP ruleset servers in patch 1.8. I am sure many of those people could have just rolled on one of the lesser populated servers. But it wasn’t server population that was the issue, it was the ruleset of the servers. That’s what made the difference.
In my case, I play on the Oceanic servers. There are currently only 3 PvP servers and 1 PvE server to chose from. Since I don’t want to play on a PvP server, which “dead” server am I supposed to re-roll on? It’s not like I have a grand amount of choices here. But again, you are missing the point. The point is not about the population of the servers, it is about setting up some RPG servers that have no cash shop conveniences on them for us “old school” gamers who still enjoy playing an MMORPG.
And I don’t think it’s as few of players as you would like to think. This post has 5.7K likes with 605 of those being on the first post alone. I haven’t seen that sort of response on any of the pro-boost posts yet. And I am certain that there are more people who like and prefer to use the cash shop conveniences than there are those that don’t. We may not be the majority any more but I definitely think there are more than just “one or two” of us who would like to play on a fresh, new TBC Classic server with no cash shop conveniences on them. 
Thanks.