Because the basic mechanics and features aren’t in alms.
I do think that’s a fair answer to your question.
You can’t really build a good game unless you get the basic features, the general ideas of the game in place, then build on them.
And you can add new mechanics indefinitely, but at some point you have consider the resource drains. If you add a feature that’s totally cool, but less than 1% of the players use, and takes as much effort to maintain as a feature that 30% of players use (or god help us, 100% of players, and this has happened!), then you can’t really justify your use of resources.
This came up for Blizzard, interestingly, with raiding. With Naxx, in Vanilla, less than 5% of players even saw the inside of Naxx. A smaller number downed a boss. And Naxx was a huge art investment, with 13 tightly-tuned bosses with tons of loot. It was such a waste, that they brought Naxx back as an up-leveled raid in 2008, in Wrath of the Lich King, and relatively few people complained - because honestly, how many of them had even been there?
A similar issue occurred with Sunwell in TBC, but unfortunately it didn’t have convenient lore to allow it to be re-done. Even few people played that, though, whilst it was current content (1%? I think Blizzard said).
Blizzard loved raiding, but they realized this wasn’t a rational or justifiable use of resources. So they did three things:
Re-used Naxx. Recycling! Good citizens!
Made WotLK raids much more accessible, with 10 and 25-person difficulties, and later Hard Modes and Heroic Modes. Bosses were generally tuned to at least allow people to get some way into a raid before shit got too real.
Towards the end of Cataclysm, introduced “Looking for Raid”, to ensure loads of people got to see raids.
And by Pandaria, figures were up from 5-20% of people seeing a raid to 40-60%! Which is rather more similar to Molten Core and BWL in Vanilla (40%-ish for MC, 30%-ish for BWL, IIRC). So now, phew, sigh of relief, the resource usage was entirely justifiable.