Because of Bandwidth. Of course 1080p is the best quality out there (i am not talking about 4k here) but considering the difference between quality it would save a lot of money to broadcast a signel with 1080i quality. It takes almost half space than 1080p.
Many people say 720p and 1080i are almost same as 1080 interlaced video shows half the resolution per frame than progressive. But in tatasky stb you can see loss of some corners part of video in 720p compare to 1080i this is because even if its interlaced its 1080 and 2 frame of 1080i makes 1 frame of 1080p which is bigger than 720p. So for me its 1080i best than 720p.
tv channels are because of reason that 1080i have half information per frame than 1080p, 1080i channels have smaller size than 1080p channels and useful where size matters like satellite communication where because of limited space it concerns.
if 1080p requires a lot of bandwidth then just think how much bandwidth is required for 4K n 8K TV brands should not even think of 16K for atleast 20 more years as 1080p itself is not in use (For dth n cable that is)