ubiquity - Why don't I have the option ''Install Ubuntu alongside'' my other systems?

Why don't I have this option?

here's my disk layout

sudo fdisk -l

Disk /dev/sda: 640.1 GB, 640135028736 bytes

255 heads, 63 sectors/track, 77825 cylinders, total 1250263728 sectors
Units = sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes
Disk identifier: 0x5b53cc54

   Device Boot      Start         End      Blocks   Id  System
/dev/sda1   *        2048      409599      203776    7  HPFS/NTFS/exFAT
/dev/sda2          409600  1153767021   576678711    7  HPFS/NTFS/exFAT
/dev/sda3      1216962560  1250050047    16543744    7  HPFS/NTFS/exFAT

/dev/sda4      1250050048  1250261679      105816    c  W95 FAT32 (LBA)

Disk /dev/sdb: 4005 MB, 4005527552 bytes
32 heads, 63 sectors/track, 3880 cylinders, total 7823296 sectors
Units = sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes
Disk identifier: 0x20d8782d

   Device Boot      Start         End      Blocks   Id  System

/dev/sdb1   *          63     7822079     3911008+   c  W95 FAT32 (LBA)