That system definitely has problems (it's terrible and sucks in many places), but it's a system of much larger capability and complexity than Palantir.
Comparing Palantir to it is like comparing a wheel to a an entire transport system. Palantir could comfortably fit in as a single capabilty in DCGS-A (and probably do a better job than the stuff currently filling in that role), but it could never replace it.
It's basically just a federated search, mediocre map and halfway decent link-analysis tool. But ask it to do anything even remotely outside of those three things and you're basically dead in the water as it doesn't offer anything relevant to all the other millions of things DCGS-A provides. DCGS-A is more than just some analytic tools.
The reason it's not part of DCGS-A is very political and complex and as much Palantir's doing as the Army's, but the answer is that the components that are on DCGS-A that basically do what Palantir does were selected because they're cheaper over the long run, even if a bit clunky. For example, on the client side, there's presently a forward looking mandate from big Army to drop flash and java clients (because they're an IT administrative menace). Palantir's front end is Java.
