There are many historical and contemporary reasons for this.
First of all, Rishikeshhosts a plethora of Yoga institutions in the present time. On an average, almost all the Yoga institutions in Rishikesh, are of international level, where both Indian and foreign Yogis come for the practice. Some of the acclaimed institutions are International Yoga Alliance, World Yoga Alliance, American Yoga Alliance, Indian Yoga Alliance, all these big institutions are unmatched, and the Yoga centers in Rishikesh follow the syllabus and ways of such big institutions to train the yogis here.
(Courtesy: tripsavvy.com)
Secondly, Yoga has a history of its own associated with Rishikesh. It is a traditional practice there which is being followed since the ancient times. Talking of the more recent times, there is Maharishi Mahesh Yogi, Swami SatyanandSaraswati, etc, who laid the foundation of Yoga practices and full-fledged Yoga centers in Rishikesh in the 90s. Before that there were just huts that Yogis and Yoga gurus used to have for the practice of Yoga. But now, Rishikesh is a hub with Yoga activities taking place in almost every nook and corner.
(Courtesy: ashtangayogattc.com)
Also, Yoga is far from the superficial Yoga practices that are popular at other places. Rishikesh hosts yoga in its truest and purest form.
Lastly, Rishikesh is famous as the best yoga destination. At other places, you may get good Yoga teachers and trainers, but you won’t get the appropriate environment like there is in Rishikesh.
Rishikesh is the major destination for the tradition of Yoga.