Why 720i?

[hkl8324]

Why on the earth there is a broadcast mode known as 720i?

1.All HDTV support progressive mode
2.Unlike 1080p, 720p have no bandwidth problem as I know
3.Deintelacing a interlaced video make the quality worse

It there any technological reason that answers my question of "Why 720i"?

Edit:
(O...I checked, there is no 720i broadcast in the world, only same 1080i>720i video floating on the net....sorry for my ignorance)

[Video Dude]

http://en.wikipedia.org/wiki/720i

[reepa]

According to A/53E ATSC Digital Television Standard, Revision E with Amendment No. 1 the following resolutions and frame rates are supported:

1920x1080 progressive
23.976 24 29.97 30

1920x1080 interlaced
29.97 30

1280x720 progressive
23.976 24 29.97 30 59.94 60

And a bunch more 480-line standards (640x480, 704x480 at different frame rates and aspect ratios).