Because Satva Guna helps in approaching spirituality thought it doesn't lead one to Moksha.
See the nature of three Gunas as described in Bhagavag Gita
14.11 When the illumination that is knowledge radiates in this body through all the doors (of the senses), then one should know that sattva has increased greatly.
14.12 O best of the Bharata dynasty, when rajas becomes predominant, these come into being: avarice, movement, undertaking of actions, unrest and hankering.
14.13 O descendant of the Kuru dynasty, when tamas predominates these surely [i.e. without exception.-M.S.] come into being: non-discrimination and inactivity, inadvertence and delusion.
Also see the next birth according to the dominance of particular Guna while departure:
Satva : attains the taintless worlds of those who know the highest
Rajas : born among people attached to activity
Tamas : birth among the stupid species.
14.18 People who conform to sattva go higher up; those who conform to rajas stay in the middle; those who conform to tamas, who conform to the actions of the lowest ality, go down.
So, it's very obvious that there are very less chances of obtaining true knowledge for the followers of Rajas and Tamas Gunas. Agree that Satva Guna will not lead one to Moksha, for approaching Moksha, one needs to go beyond all three Gunas. As preached in 14.6th verse : "it binds through attachment to happiness and attachment to knowledge." However it is better than Rajas and Tamas as it helps in purifying mind and in becoming eligible for one to obtain the true knowledge.
Q. Why can't a person with RAjasika or TAmasika nature do "Karma yoga" with SakAma Karma & attain Moksha?
As mentioned above, Rajasika or Tamasika mind is not eligible to understand or obtain the right knowledge that is required for Moksha. So, regarding your queries you can consider that Rajasika or Tamasika Sakama Karma Yoga is wrong tool which will never lead one to Moksha and comparatively Satviak Nishkama Karma is preferable (right tool) which can lead one to the Moksha.