Because of New Jersey's contract status, Mercer's production and trajectory vs. Meier at the same ages, and Meier's contract.
Let's look at Mercer's statistics:
Mercer 2021-2022 (Age 20): 82 GP, 17G 25A 42Pts (0.5 PPG)
Mercer 2022-2023 (Age 21): 59 GP, 19G 20A 39Pts (0.7 PPG) [Season Incomplete, on pace for 82GP]
Mercer 2023-2024 (Age 22): ?
Meier 2016-2017 (Age 20): 34 GP, 3G 3A 6Pts
Meier 2017-2018 (Age 21): 81 GP, 21G 15A 36Pts (0.4 PPG)
Meier 2018-2019 (Age 22): 78 GP, 30G 36A 66Pts (0.8 PPG)
Mercer's ahead of Meier's development curve at the same ages. He easily could match Meier's Age 22 production when he's the same age (especially if he's in the top-6 and not on the third line, which is where he's spent most of the last two seasons).
So, projecting outwards to next season, it's not at all clear that Meier's production would be far and away better than Mercer's. He'd also cost between $8M-$10M, while Mercer would still be on his ELC for another year. And we'd have to trade other assets along with Mercer to bring back Meier. Plus Mercer is right-handed, which has value, and can play center, which has value. We also know Mercer already works with our existing core, while bringing in a new star is always potentially risky for a locker room. Mercer's contract demands may also complicate Bratt's contract demands (since Bratt is not going to want to be paid less than Mercer).
These are all problems that are worth dealing with if you're bringing in an obvious long-term improvement for the top-6. But Meier isn't an obvious long-term improvement over Mercer.
And as others have said above, the Devils are not in any rush to compete this season necessarily. Our focus is still for the future, not for right now. There's a decent chance Meier hits UFA this offseason, or in a year after he takes his 1 year QO. We can always pursue him then.