The most common reason is that the driver vendor has mislabeled items within the driver which indicated to Smart Driver Updater that it supported your particular hardware and operating system when it does not. When a driver fails to install Smart Driver Updater marks it as "pending update".

We are constantly updating our driver database to ensure that we only offer the best available drivers for every device. When we identify problem drivers that fail to install we remove them from our database and immediately focus on finding better drivers for those devices.