Because they want people to tune in week to week and eventually come back for Season 3.
The first game/season had a bright center to it that people connected with: Joel and Ellie and their relationship. There's a reason Season 1 is largely a copy/paste job: it worked. I haven't played the second game, but we all know the second game killed that center - literally. What I've gathered from the online discussions is that about half the people who played the game didn't like it much. Chief amongst their complaints was that they were left with a fairly unlikeable lead in TLOUP2's version of Ellie. In fact, I've seen far more people enjoy Abby's half than Ellie's half. That's a real problem for the TV show because, unlike the game - where players had already put down their money upfront - the TV show has to keep people coming back. You can't expect people to stick around for hours and hours of grimdark with a largely unlikeable lead. Clearly, they wanted to avoid the whiplash that gamers were smacked with. So, to actually hold on to their audience, I suspect Mazin chose to keep Ellie closer to Season 1 of the show, someone returning viewers might actually recognise.
Of course, it would appear they're still trying to follow the structure of TLOUP2, where Season 3 will be told from Abby's perspective. I don't think that's a good idea, frankly. Once people realise the characters from the first two seasons aren't coming back for a long time, they may drop the show. I imagine they're aware of that, so I'd expect them to truncate Abby's story to catch it up quickly and then tack the end of the game onto the end of Season 3. And I can't imagine anyone will be happy when its said and done.
