There are 3 reasons I can see.

1:
It’s a huge change, and they did a dual-implementation however long ago - where they store it in the active legacy system, and also store it in the new inactive system ready for when the switch is flipped.
Do a year of this, changeover flawlessly, and suck up the outrage over lost data.

2:
It’s a maybe not such a large change, but the data processing for it is expensive. So, however long is an acceptable cost, and its balanced against user outrage

3:
There was a TOS change. So data before that date can’t be sold/monetised, whereas after that date has value.
So, drop the data that costs money, keep the data that can be monetised.

Whatever, it’s bullshit.
Glad I left. And glad Lemmy is cool