If you look at adoption patterns at the enteprise level, wikis are an unusual technology.
Most enterprise IT deployments are all about standardization and scale. A purchasing system reduces procurement costs by aggregating purchasing power across business units. A corporate finance system enables aggregation and standardization of accounting ledgers company-wide. An HR system enables standardization and cross-calibration of roles, titles, compensation bands, utilization rates, etc., across the company. The benefits of these systems are all about leveraging enterprise-wide scale benefits of aggregation, standardization, and cross-calibration.
As a result, these systems are almost always implemented top-down. Centralized functions (Purchasing, Finance, HR, etc.) partner with IT to mandate the implementation of these enterprise-wide systems. There's usually a struggle with individual business units, who insist that their needs are unique and Corporate doesn't understand the realities of life in the trenches. These traditional IT systems are implemented when the interests of the overall firm trump the interests of the individual business units.
Wiki as a technology tends to spread very differently across an enterprise. Wikis usually come into an enterprise through local, grassroots efforts. A group tries a wiki, likes it, and starts seeing business value. Other groups get wind of it and try it for themselves. The impetus is bottom-up from the business rather than top-down from the corporate center. It's a very different adoption pattern.
What accounts for the difference? I see three factors.
First, wikis are not a scale play. A single business unit, a single team, even a single person can derive business value from using a wiki. Of course the network effects are exponentially greater at larger scale, but there's a lot of benefit even without the scale.
Second, wikis are not a standardization play. Traditional IT systems are all about trying to limit variation and get everyone to do things the same way (often for good reason). Wikis area all about morphing, molding, and adapting to the way people and groups want to work. So the grassroots appeal is not surprising.
Finally it's difficult to force someone to use a wiki. It's relatively easy to force compliance around a Purchasing, HR, or Finance system: simply mandate usage and take away the alternatives. But the alternative to wikis is email, and who's going to take that away? People will use wikis only if they want to, so the adoption has to come from front-line workers freely choosing the wiki over other alternatives equally available to them.
Addendum 3/29: Since I posted this last night, I realized I overlooked the most obvious way in which wikis are different from traditional enterprise-wide IT deployments: economics. Thanks to SaaS delivery models like Socialtext and open source products, individual departments and business units can implement wikis quickly and cheaply. They don't need enterprise-level IT to get in the game. Over the long run, most companies will benefit from a more holistic approach to wikis, but the economics help explain why in many companies it's the individual business groups who are leading the charge.