Because I wanted to, later making the PED suit (with corrupted forms).

Visually, the suit is arguably the first dramatic departure from Samus'
traditional Suit appearance since the addition of the Varia's
shoulderpads in Metroid II: Return of Samus.
Most noticeably it features darker, more muted colors than previous
upgrades, and replaces the traditional spherical features on her
shoulders with rounded wheel- or disc-like protrusions, and two pointed
struts protrude from the back, but these seem to be meant to support the
Gravity Boost once acquired. Upon acquisition of the Gravity Boost, the Dark Suit's bulk increases slightly at the rear of the torso. At this point the suit rather resembles a Luminoth,
with a near-spherical torso, minimal neck showing, and various red orbs
dotting the exterior. It also resembles the various visual aspects of
Ing possession, especially possessed technology such as Quadraxis and
the Ingsmashers. The visor's shape also changes, making it closer in
appearance to Dark Samus'.
It is also worth noting that these red orbs, prior to the acquisition
of the Gravity Boost, are featured on her back in a triad closely
resembling the Dark Amorbis' eyes. This is similar to the three diodes
on the back of the Fusion Suit, which is said to resemble Metroid nuclei.

Source:http://metroid.wikia.com/wiki/Dark_Suit