Why is sarcoma more prevalent and common in right heart?
Ewing's sarcoma is a bone cancer. As such, it does not arise as a primary tumor in the heart.

Ewing's sarcoma does metastasize. Like any metastatic cancer, it seeds along it's venous return to the heart, "taking root" in suitable tissue. 

Cardiac metastases of Ewing's sarcoma are exceedingly rare, with only a few reported cases. Since all blood returns to the right side of the heart first, then the lungs, this is where distant metastases of Ewing's sarcoma are found. The side doesn't matter; it's never good.

Primary heart tumors do occur, about 1/20th as often as metastatic tumors. Of these, myxomas are the most common. Of sarcomatous types, which are very rare, they can occur anywhere.

Ewing's Sarcoma
Metastatic Ewing's Sarcoma to the Right Ventricle
Primary Malignant Sarcomas of the Heart and Great Vessels in Adult Patients — A Single-Center Experience
Echocardiographic and pathologic characteristics of primary cardiac tumors: a study of 149 cases
Primary cardiac sarcomas: an immunohistochemical and grading study with long-term follow-up of 24 cases