The basic reason, the decision was not illegal when it was made and seniority systems have their own treatment under Title VII, which recognizes their importance in maintaining stability. That was the same reason the Lily Ledbetter Act was not relevant, even though the Supreme Court had allowed supplemental briefing on its potential impact. Quoting from the statute, the Court answered plaintiff's argument that it revived her cause of action:
For the reasons already discussed, AT&T’s pre-PDA decision not to award Hulteen service credit for pregnancy leave was not discriminatory, with the consequence that Hulteen has not been “affected by application of a discriminatory compensation decision or other practice.” §3(A), 123 Stat. 6.
If the case had gone the other way, it is possible to imagine how it could have had broad ramifications. However, given its narrow holding, it seems unlikely to be very important beyond its impact on the parties.