How to use template_preprocess_views_view to pass a variable to a views tpl file
As you would also know, a view has several templates, which can vary depending on the style, for example:


Display output: views-view.tpl.php
Style output: views-view-STYLE.tpl.php
Row style output: views-view-fields.tpl.php
Field content: views-view-field.tpl.php


etc.

Each of those templates has its own preprocess function. What you do in your code is only affecting the display output since you are preprocessing only the views-view.tpl.php (template_preprocess_views_view)

To find out which template files are available to your specific view, you can go to view edit page and under 'Advanced' section at the end of the column click on 'Theme: Information'. You will see which template files your view is using (in bold) and which template files you would use to override them.

The preprocess functions should be named after the template file, replacing the dashes with underscores. For example for views-view-STYLE.tpl.php would be template_preprocess_views_view_STYLE().