A few things:
1. Money - by not offering the second Watchtower to the public, they can save huge amounts in printing costs.
2. Compatibility with Awake. It must have been difficult to offer a second Watchtower without an Awake to get people interested; much easier now that there's a one-to-one correspondence again
3. Embarrassment. The Watchtower study articles are sometimes so wacky as to embarrass even JWs, especially when presenting them to members of the public. Much better to stick all the more ludicrous stuff in a members-only version.
4. Repositioning - Awake has already been revamped; making a snazzy new Watchtower might boost consumer interest for a while (although I'm not sure they care so much about that).
I think some or all of the above are the reasons, and there are also likely to be ancillary benefits. For example, JWs always get excited when there are new releases or structural changes. Somehow, it means the end is nearer - but then doesn't everything?