Well it was only 5.30pm so it's not like it was late night or anything, just the family-friendly/anime slot just before primetime. Going by memory, there was a recent episode of Sci-fi Japan (a great youtube show, in case you haven't seen it yet) about the Heisei revival of Ultraman, where they mentioned that the Showa era shows had all aired on Saturday evening primetime spots, and Tiga also ultimately did (albeit on a different channel to all the Showa ones). The implication was very much that Tsuburaya regard it as being the natural home for the franchise. I've heard before that the Ultra franchise shows are regarded in Japan more as family shows rather than a straight-up kids shows (kind of like how Doctor Who is very kid-friendly, but is regarded here in the UK as whole-family viewing and airs at around 6.30-7pm on Saturday evenings) so it wouldn't be unexpected in that timeslot.