Here's how four standard contemporary versions render 1 Pet 3:18:
being put to death in the flesh but made alive in the spirit (ESV).
having been put to death in the flesh, but made alive in the spirit (NASB).
He was put to death in the flesh, but made alive in the spirit (NRSV).
He was put to death in the body but made alive in the Spirit (NIV).
Here's a notable exception:
being put to death in the flesh but made alive by the Spirit (NKJV).
Of course, that's a revision of the classic 17C version.
Since I don't sit on translation committees, I don't know why they generally render the phrase that way, but here's what two commentators say, which may reflect the thinking of translation committees:
Jobes says that given the parallel syntax, the two datives should be rendered the same way (240). Fobes agrees.
Since the instrumental construction ("by") is not consistent for both, that, in turn, selects for a locative construction ("in"). Hence, as Forbes goes on to say:
Christ was put to death in the realm of the flesh but made alive in the realm of the Spirit (123).
By further implication:
It is unclear whether "Spirit" should be capitalized or not. Having rejected the instrumental sense for pneumati and opted for a locative of sphere (i.e., mode/realm of existence), the parallel with sarki probably favors a lower case "spirit" (123).
Jobes is a fine Greek scholar. An expert on the LXX. Likewise, the EGGNT series (to which Forbes contributes) is specifically designed to interact with the finer points of the Greek text. But it seems to me that there are two basic problems with their analysis:
i) Both Jobes and Forbes agree that "made alive" alludes to the Resurrection. However, to say that Christ was raised in the "realm" or "mode" of the Spirit is an obscure and frankly misleading way to describe a physical resurrection.
ii) There's also an issue regarding translation theory. The fact that the Greek construction is parallel doesn't mean the English construction should be parallel–for the obvious reason that Greek and English aren't the same.
One Greek dative can have two different senses (locative, instrumental), where as it takes two different English prepositions to express each sense. Therefore, a Greek writer can use the same word to express two different relations. He doesn't have to sacrifice verbal symmetry to differentiate the sense.
But verbal symmetry doesn't require semantic symmetry, precisely because the Greek is already flexible in that regard. A Greek writer could intend a locative sense in the first clause, but an instrumental sense in the second clause. He can do that without resorting to two different constructions–and for stylistic reasons, he might prefer the elegance of using the same construction in both instances, even though each carries a different nuance.
To take a comparison: English translators are confronted with a tough choice in Jn 3:3. For John, this is likely a double entendre. The same word can both mean "again" (or "anew") and "above." Both senses fit the context. Both senses are probably intended.
But we lack an English synonym with the same semantic range. Therefore, a translator must make an arbitrary judgment call. At best, he can give the alternate rendering in a footnote. By contrast, John wasn't confronted with that false dichotomy.
iii) For theological reasons, I think we should render the phrase "made alive by the Spirit." Christ was raised from the dead by divine agency. That's widely attested in NT theology.