Why does SSRS not update a report's description?
This is a &quot;known&quot; issue/feature according to Microsoft.  Descriptions, parameter defaults, subscriptions, etc. all fall under report &quot;meta data&quot; and are maintained separately on the Report Manager server.  The only way to get these to change on the Report Manager is to either manipulate them manually or delete the report and upload anew.  The links below deal specifically with this issue in regards to report parameters, but the same is true for descriptions.
Here is an excerpt from a Technet Article regarding parameters

Parameter properties that you set in Report Manager for a published report are generally preserved if you republish the report definition from Report Designer. If the report definition is republished as the same report, and parameter names and data types remain the same, your property settings are retained. If you add or delete parameters in the report definition, or change the data type or name of an existing parameter, you may need to change the parameter properties in the published report.

This is a related Connect issue

Parameter defaults do not get updated when re-deploying existing reports. These either have to be updated manually or the reports deleted and re-deployed. The latter regenerates all report ID's (GUID's) and makes traking usage from the ExecutionLog more difficult.
This is explained here (link) as being by design however I can't envisage parameter defaults and prompts being maintaned by an administrator.
An override mechanism similar to OverwriteDataSources should be added to Reporting Services projects to allow deploymnent from Visual Studio.