Many years prior to the rule of Dashrath, Ayodhya was ruled by Trishanku, an ancestor of Rāma. Though virtuous, this king had an amusing yet unusual desire contained in his heart. He wanted to ascend the heavens along with his body and flesh, a feat noone had achieved before. He communicated his wish to his guru and priest Rishi Vashishtha. However, the wise Rishi refused to fulfill this audacious desire. A mortal ascending heaven along with his physical body was antithetical to the laws and ethos of nature, and could also disrupt the subtle balance of the universe. Trishanku, however, did not relent. With his wish in his heart, he arrived at the door of Vishwāmitra. At that time, Vishwāmitra was not yet a Brahmarishi. Filled with vain hubris and in order to prove his prowess, Vishwāmitra agreed to help Trishanku reach the heaven with his body.
Vishwāmitra channeled all his power, he had accumulated through his Tapa(तप). And Trishanku with the help of this power, rose upwards towards the sky, only to end his journey at the gateway of the Heaven. Trishanku felt his desire fructify, but the demigods were not pleased with Trishanku. They complained Indra about Trishanku’s audacious act. Indra grew enraged and as a punishment banished Trishanku from the heavens down to the earth. Trishanku therefore began to fall down towards the earth. When Vishwāmitra saw Trishanku falling down like an inanimate object, he realised that his maneuver had failed. To make up for his failure, Vishwāmitra made Trishanku halt mid air and made a Loka for him to dwell.
Therefore, Trishanku was doomed to always remain mid air.
Source: Sarg 60, Bāla kānda, Vālmiki Rāmāyana