Because the startup costs to get a lot of their customers actually set up on their platform are too high, so free trials for say, 14 days, are out of the question.

For example, take a look at http://www.curalate.com/ which uses some really amazing technology to track a customer's brand imagery across (almost) the entire web. Naturally, most of these types of services are usually $300++/month, so again, it's all just about mitigating setup costs for the provider.

Hope this helps :)