Manufacturers, with the notable exception of Leica, apparently don't consider digital rangefinder cameras to be worth making.
Film rangefinders came in quite an array of types and families, from pocket-sized fixed-lens cameras to entire interchangeable lens systems.
Why haven't they survived into the age of digital photography, in the way that SLRs and compact cameras have?
It isn't that rangefinders have struggled in the era of digital. It's that rangefinders struggled to survive the era of the SLR. The SLR started putting the rangefinder to bed long before digital came along. Canon produced its last rangefinder in the Canon 7 (introduced in 1961).
Leica has gone upstream with their product and target market. They make highly capable, very pricey cameras targeted to a very specific type of consumer. The rest of the brands went a bit more mass market.
Today, the mirrorless and compacts fit the same need as the rangefinder did.
Want a small, pocket-friendly camera for snapshots? Digital compact.
Want a lightweight, more capable camera for your travel photos? Mirrorless.
It's all about market needs. A huge chunk of the rangefinder market went to film compacts or SLRs as their needs dictated. When digital came along, they went to digital compacts or mirrorless. There's simply no space in the mass market for a digital rangefinder. Leica is niche – there's no market room for the mass market brands to either get in on that niche or attempt to expand it.
I don't remember seeing many popular rangefinder cameras at the dawn of the digital era. The main reason for the demise isn't digital-vs-film, but the pervasiveness of zoom lenses, for which they are not very well suited (you would need to couple the view finder to the zoom of the lens) and even worse interchangeable zoom lenses.
Mirrorless fills this niche in a lot of ways, and particularly as EVFs and sensors have gotten better, the downsides to that have narrowed.
The "rangefinder" in "rangefinder cameras" refers to these cameras' unique system of manual focus. I've used a digital Leica a little bit, and at least without a lot of practice, my impression is that while this may be superior to SLR manual focus, it's still... manual focus. Of course, many great photographs have been and can be taken that way, but in general, consumers of all sorts (casual shooters, enthusiasts, and professional) want autofocus.
As xenoid notes, the rangefinder system doesn't work well with zooms. In fact, beyond that, it's kind of awkward with long lenses even if they don't zoom, and also incredibly difficult for macro. A through-the-lens viewfinder system (whether SLR or mirrorless) is superior in all of these cases.
Rangefinder cameras are compact because they do not need extra room for a mirror. The mirrorlessness is accomplished by having one optical path for a photosensitive medium (film here) and another for the photographer.
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Compact digital cameras also lack the mirror, but there is no need for extra optics whatsoever. You can dump the data acquired on the chip continuously and feed it to the finder or screen. If the shutter is released, the dump is redirected to memory.
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As mattdm notes, rangefinders use a focusing mechanism based on parallax. Several opine that any popularity rangefinders might have had were for their compact size, not their focusing mechanism. (mattdm, Hueco, Crowley)
However, there is at least enough demand for Leica to continue making and selling rangefinders for a huge premium, despite their relatively low volume. The true level of demand and the price the market would bear is unknown because Leica has a monopoly on digital rangefinders.
The popularity of rangefinder-style cameras, made by Olympus, FujiFilm, and others, demonstrate that there is demand for the rangefinder look, for which people are willing to pay a modest premium. These cameras are externally modeled upon classic rangefinder cameras, but use autofocus and TTL technologies. If a rangefinder focusing aid were added to one of these cameras at a reasonable price, it would attract those who are fond of true rangefinders.
