It's mainly to keep a consistent and uniform style across the Wiki. A number of other Wikis (particularly ones that concern franchises that deal with many events taking place over a number of years, most notably Wookieepedia) employ this style as well. Fellow admin Pyramidhead first made this suggestion a few years ago as such:
"I think we should consider using past tense across every article, including the intro. The idea is that we're writing about past events infinite years in the future, so that everything in the story already happened - after all, the person reading these could have just started watching the first season or they could be completely caught up; it's sort of misleading and confusing to start out as if someone's alive and kicking when they haven't been seen in 30 years. Wherever Leo is he's not perpetually a trucker who Truman can't catch, etc.
This would lead to some weird phrasing on certain articles - it's very odd to talk about real cities and states as though they were this and that - but it's much more consistent IMO. The article immediately jumps into past tense anyway so it's not that big of a change."
The suggestion was before Season 3 ended, so to add to that, since the finale, there is no indication of what the "present" year is. So it's easiest to refer to everyone and everything within the narrative in the past.
