When you speak of the "larger threads", I assume you are talking about e eGo cone threading on the outside of the 510 connection on eGo batteries and other devices. Some big battery mods have them, and some do not.
The reason (I assume) that a device like the ProVari, and other high end VV and mech mods don't have ego threading probably has a couple reasons. First and foremost, there are only a handful of devices that utilize the eGo threading(mainly eVods/T3s) and most people don't use these types of attys on devices like the ProVari. The other reason, I suspect, is that on the ProVari specifically, not having eGo threading allows for a large, yet shallow drip well that is a lot easier to clean than a device with eGo threads and a very deep well (like the VAMO)
Also, adapters are cheap, easy enough to use, and work well. If you really want to put an eVod or MT3 on your ProVari, it'll cost you $3 for a 510-eGo adapter.