HTML rendering extension displays multicolumn report as a single-column and that behavior is by design due to the fact that html is not a page oriented format. Please, review the help section Design Considerations for Report Rendering
for more information about rendering extensions.
Also you can review the topic Design Considerations for HTML Rendering
for specific information about what to keep in mind when designing reports that will be used with HTML extension.
All the best,
the Telerik team