Why my custom form doesn't work?

Actually this works as designed, and can be changed by a setting.

Clientside Validation does not validate hidden fields, as we do not know if the user will ever get to see the hidden field (this is to support modules like Webform Conditional)

You can either change this for every form, or only for your custom form:
For every form:

1. Go to admin/config/validation/clientside_validation.
2. Click on 'Default Settings'
3. Scroll down to the 'Hidden fields and tabs' fieldset
4. Check 'validate hidden fields':
   

For only one form:
Either follow the same steps as described above and fill in your form id in the textarea:

Or follow these steps:

1. Go to admin/config/validation/clientside_validation.
2. Click on 'Custom Froms'
3. Add the right form id:
   
4. Click edit next to your form id:
   
5. Check 'Override default options for Hidden fields and vertical tabs' and you will see a new fieldset with the settings:
   
6. Check 'Validate hidden fields' and save:
   
7. Click enable next to your form id and confirm:
   

And you're all set. This way the hidden fields will be validated as well.

PS:

1. You can do the exact same thing for content types as well:
2. Go to admin/config/validation/clientside_validation/content-types
3. Click 'create' next to your content type
4. Follow the same steps as above starting from 5.