They work well in systems with tight physical coupling. They work well in N-tier applications where the developer has control over the entire scope of the application. They are able to take advantage of proprietary technologies related to operating system, network and software development. They are quite efficient if used properly. They have had a decade to mature.
Many popular tools provide development and deployment support.
And a very important consideration is that traditional components are quite familiar to object-oriented developers. Unfortunately, many of the most popular OO technologies and languages have taken the narrowest view of such classification. It becomes harder and harder for traditional component technologies to model such diversity and fluidity of data in a manageable way. The promise of XML-related technology to deal with this problem is one of the reasons XML has exploded onto the scene with such force. Most component tools provide some means of finding components for a particular purpose in order to extend an application after initial deployment.