When the plugin imported your backup, it checked if the webserver supported pretty permalinks.
The plugin detected that there is no support for pretty permalinks in your webserver and set the permalinks structure to the default type.

How to enable pretty permalinks in Apache:

• Enable mod_rewrite module – How to enable mod_rewrite in Apache
• Set AllowOverride All in your Apache site config – How to set AllowOverride to All in apache config
• Login to wp-admin dashboard. In the Settings → Permalinks Screen, choose one of the more common permalink structures or enter your own in the “Custom structure” field using the structure tags.

How to enable pretty permalinks in IIS:

• enable URL Rewrite 1.1 module – How to enable URL Rewrite in ISS
• Login to wp-admin dashboard. In the Settings → Permalinks Screen, choose one of the more common permalink structures or enter your own in the “Custom structure” field using the structure tags.