Here are a few possible reasons:
1. The intelligence engine is still searching - have a look again in a couple of minutes, you may need to press the 'Refresh' button
2. The intelligence engine didn't realize the website is relevant to your Canvas - adding some more Elements and Relations that explain your context (meaning) better may help. See: How are the scores for suggestions calculated?
3. 'Semantic differences' - basically the intelligence engine only uses each page on the web once so it may have interpreted it as having a different semantic meaning.
You are free at any time to manually create an Element for the website you want and add a description; you don't have to wait for it to be suggested.