Maybe because for Mercy, that would be a direct buff, not a quality of life change, whereas the changes to Ana were either for an entirely separate game mode, or an actual quality of life change that players can toggle to have on or off, both with their advantages and disadvantages.
I’m not a fan of the phrase “keep up” here, I don’t feel it’s very fair of a statement. Like, a big reason why Mercy isn’t as viable as Ana right now is due to the meta. With Rein being back as the powerhouse tank, Ana is a natural compliment, and Lucio eats up the other slot. The synergy is really strong and the composition doesn’t have room for Mercy if the team wants to play what’s most effective. This doesn’t mean Mercy is underpowered, but that what she offers currently is going to have a hard time being supported by the rest of the team if they’re building themselves towards the meta.
It isn’t that Mercy is missing something huge from her kit, or that her numbers just aren’t enough. The fact is that in your go-to game where teams want to win as much as possible, currently, there isn’t as good of a place for Mercy as there are other supports.
But even that is kinda wrong. Realistically, across a wide range of ranks, you can still make Mercy work by executing her well enough with a team capable of benefiting from her. It’s not like meta comps are utterly unbeatable and you have to play it in order to win… but it’s unfortunate that from the lens of an average player, that feels like what has to happen.
Look, if you enjoy playing Mercy, she’s not so weak that you have to stop playing her because of her weakness. What will more likely push someone off of Mercy is that the rest of the team is trying to run meta, and when that meta doesn’t fit Mercy, she naturally becomes a less desirable pick and one is more likely to compliment their team with a different hero than try to shoehorn one that doesn’t.
Mercy isn’t really a problem hero one way or another. She doesn’t overperform nor does she drastically underperform, especially when all ranks are accounted for. That’s a different story for Ana, and why she sees more attention given to her. But really, I wouldn’t really look at this Nano Boost change as some sort of favoritism among Blizzard, it’s a trait that should have been in the game for a long while and it doesn’t imply at all that they’re ignoring Mercy. If Mercy isn’t in need of some major shake-up, she isn’t going to get some major shake-up.