Because the outlook of some blocks may not be the same as some players intended in their builds, also it affects new ones as players won't know what it'll look like after Christmas
You are not forced to use it. To disable it just head to your multiplayer tab after opening the game, click on SMPEarth then down the bottom click edit. There will then be a button that says "Server Resource Packs: Enabled", just click it once and it should change to "Server Resource Packs: Disabled" and this will disable the new texture pack.