A couple of reasons why not.
First is the focus problem. A longer lens has shorter DOF in general, so you either have to make the long lenses very slow, or you have to have a wider rangefinder base (and thus a larger camera body) to focus more precisely. This is also part of the reason why MF rangefinder lenses aren't particuarly fast. The other part of the reason for that is that the lenses have to cover a larger format and would need to be inconveniently large to get an aperture like f:1.4.
The other is the viewfinder problem. A wider focal length range requires a more complicated viewfinder, either with more framelines or with a more complex zoom arrangement. With very wide lenses you can use an auxiliary finder mounted on the accessory shoe, but wide lenses have wider DOF and don't require precise focusing.
Leica made it possible to use long lenses on Leica rangefinders with the Visoflex system. This inserted a periscope kind of thing between the lens and the camera body, so that you could actually focus through the lens as with an SLR. It was a bit of a kludge--better to use an SLR for what an SLR does well and a rangefinder for what a rangefinder does well.
The main reason is, it would be hard to frame properly as the user of such a camera, and it is probably hard to design a rangefinder that would track accurately enough to a long lens. The longer the lens is, the more any slight error will be magnified, along with the parallax that all rangefinders have to some degree. Then, how would you set the rangefinder optics to match a wide range of lenses? It probably would be difficult and expensive. For most types of shooting, a long lens is not needed. For long lenses, an SLR is still the better choice since you are looking through the lens and what you see is what you get (more or less).
Great camera, isn't it? I get enlargements to 16x20 from that that rival my 4x5 negatives.
Think of rangefinder as focusing with your eyes. Looking at the door handle of a car across the street and down the block, you can't really judge distance from that to the person walking on the other side of the car. Your eyes are too close together. But with the short DOF of a 210 or 300 lens, one would be in focus and other wouldn't. And you wouldn't know which. And so it is with a RF camera, whose "eyes" are even closer together than yours.
The extra viewfinder has nothing to do with it. It is the ability of the range finder...the big window with the blue/yellow tint and the little window on the other side of the camera...to distinguish a difference in what they see that makes the difference. The further apart they are, the more they are able to see something slightly differently and therefore focus. Even with the Mamiya's 150mm lens they can barely do that, which is why there are so many complaints about that lens's ability to focus and why people tell you to ignore the hyperfocal marks on the barrel.
</span><table border='0' align='center' width='95%' cellpadding='3' cellspacing='1'><tr><td>QUOTE (Aggie @ Feb 18 2003, 05:21 AM)</td></tr><tr><td id='QUOTE'>The mamiya alreadey uses axuillary finders for its 43mm, 50mm, and 210mm. </td></tr></table><span class='postcolor'>
there does even exist an auxiliary finder for the 150mm. But these finders are only for framing. They donít tell you anything about your focus. They donít have rangefinder capabilities.
BTW: having a frame within a larger view is usually taken as an advantage of the rangefinder camera. You can watch the environment of your frame/subject and release the shutter at the right moment.
</span><table border='0' align='center' width='95%' cellpadding='3' cellspacing='1'><tr><td>QUOTE (Aggie @ Feb 18 2003, 09:44 AM)</td></tr><tr><td id='QUOTE'>The extra finders also work well to preview a scene before switching out the lens. </td></tr></table><span class='postcolor'>
Some rangefinder cameras have the ability to switch the frame independent from the lens. Unfortunately, the Mamiya 7 does not have this feature. But the auxiliary finders do allow you to frame a scene without taking the camera out of the bag.
Maybe I'm becoming a bigger negative snob. 35's have the ability that I look for once in a while. But I love the bigger negatives, and wish I could find that capability in medium or large format
You can have that ability with a medium format SLR. Long lenses for medium format can get costly, but you seem to have a fair budget for photo equipment. If you also want autofocus occasionally, there are a few 645 systems that could suit your purposes.
Another option, if you want most-definitely-not-autofocus, would be to add a telephoto lens of around 400-500mm to your Wisner and a rollfilm back.
There are also some less costly options if that is a concern. I use a Bronica S2A system for 6x6 and have managed to put together an extensive kit with 40, 50, 75, 100 Leaf Shutter, 135, 200, 300, and 500mm lenses (plus a 240mm Heliar I use occasionally for portraits, half a dozen backs, and a few exotic accessories like a tilt-shift bellows) for less than the cost of an M7II with one or two lenses.