Answer
Ganesha derives the power to remove obstacles from the lotus feet of Krishna. There is a verse in the Brahma-samhita which explains that Raja Ganesha rests his two tusks beneath the lotus feet of Krishna, and from that position he receives the potency to remove obstacles throughout the three worlds — the upper, middle, and lower planetary systems. Therefore, people pray to Ganesha for the removal of impediments on the path of their progress.
Yet, Ganesha is able to remove obstacles only because he derives that power from Krishna. Therefore, instead of praying to Ganesha indirectly, one may directly pray to Krishna Himself. Specifically, devotees pray to Narasimha, the form of the Supreme Lord who removes obstacles on the path of spiritual advancement.
Devotees are generally not very interested in material progress or worldly success. True devotees do not approach Krishna merely for the removal of material obstacles. Rather, they pray especially to Nrsimhadeva to remove the obstacles that hinder their spiritual growth and devotional service.
