Old gen tiers aren't changing anymore. This is a choice by Smogon, so that way people who played the tier 10 years ago can come back and still play it today. Not everyone agrees with the policy, but when you have so many dead tiers without any way to properly test, explore or develop meta, it makes sense.
Therefore, nothing is ever added to old gen tiers, unless there's a strong reason to do so (see Farceus' legality in DPP Ubers) or if there is a strong community push and proper suspect testing (see Latias in DPP OU). Although many people love the DPP lower tiers, they certainly aren't being played enough to have a suspect voting, especially for such a large dump of meta changing mons.
On the flip side of this, things can be removed from a tier, but only when the tier is being "worked on". We see this in occasional situations like BW's recent Sleep Clause, where sleep was deemed uncompetitive and removed or when RBY moved up Vic and Rhdyon to OU and moved down Golem to UU. Also, there are tiers that are still being developed and explored by an active community, like GSC NU, where things will be removed. In this case things are backed by a hard working and active community. Things are moved to BL, because it doesn't mean anything and ultimately doesn't affect anything, despite how much some people passionately fight over the addition or removal of BL mons. Again, this is so that the tiers aren't changed, so you can play DPP UU now and in 10 years still be able to play it.