Because there's no apparent benefit to reddit to bring back long deleted comments from arbitrary users. When you can think of no motive for malicious behavior, it is unparsimonious to assume malicious behavior.
Of course there is a benefit. If users leave and remove their comments in protest, the data content available on Reddit is lowered, thereby lowering one of the IPO metrics. By un-deleting comments, the site's message count and user activity goes up, and thereby its IPO value.
It is in this case. They don't "need" the data themselves, they need Googlebot to see it to get traffic, which is their lifeblood both in general and for IPO.
Have you heard the recent popular saying that Googling things barely even works anymore unless you append reddit to the query, which tends to bring up actual information instead of SEO trash?