I don't understand why a cable company would want to keep using Tuning Adapters when this is available to them. It costs them more to use tuning adapters, so the only reason remaining to use them is one of being intentionally anticompetitive toward TiVo or other box suppliers who are not the cable company themselves.
That's the thing - they aren't for the same purpose. It isn't quite apples and oranges, but maybe oranges and grapefruit.
Tuning Adapters are exclusively for Switched Digital Video (SDV) and they, in theory, enable any UDCP host device with a USB port to tune SDV channels. In practice the only two devices I know of that support TAs are TiVo and Moxi's retail DVRs. I'm not aware of any CableCARD-enabled TVs adding support for SDV - there could be, I just don't know of them. Oh - I believe Windows Media Center also supports TAs, almost forgot about PCs. But, of all these, I believe TiVo is the only significant installed base using TAs - and 'significant' is highly relative.
The IP control channel is for OnDemand/PPV content and it talks to the SeaChange head end. (IIRC TiVo has done a deal with another head end system provider as well, but I can't recall their name. SeaChange is the big one though.) While SDV & VOD/PPV use similar technologies - SDV is basically a highly specialized form of on demand content after all, it is just transparent to the end user. But the head ends aren't the same.
While it is perfectly possible, in theory, to control SDV over an IP back channel, that's not the goal of the current system. And it would likely require a different update for the MSO than the one to support VOD - updating the SDV head end instead of the VOD head end. And keep in mind that not every CableCARD device is IP-enabled. Again, in theory any CableCARD device with USB can us a TA, no network support required - and there are a number of such devices. In practice it seems like the only devices which really use TAs also happen to be network enabled.
So, yeah, MSOs who are updating their head ends to support VOD using an IP control channel may well do the same for SDV over time. But they're disconnected from each other and enabling IP for VOD doesn't automatically get you IP for SDV. In the meantime the TA drops seamlessly into the existing infrastructure and uses all of the same protocols as an MSO STB for SDV. So it is much easier for them to deploy as it requires no head end updates and uses all of the same infrastructure.