#3. A fresh take to an Undertaker feud
One of the major problems with The Undertaker's matches is the lack of story heading into it. The last time he was involved in a solid story was in 2015 and 2016. In the latter, he faced Shane McMahon at WrestleMania 32 but he wasn't the focal point of the story - the returning Shane was.
In that case, The Undertaker was merely used as a "prop" for the storyline. He had a decent story with Brock Lesnar in 2015 but as a whole, it's been years since we've seen an enthralling Undertaker story. In 2017, the feud was simply two people telling each other "This is my yard" and nothing else.
WWE likely knows this and wanted a fresh take on an Undertaker feud. It's not often that we see him get 'humanized', but given that he now has an Instagram account, times have changed and it might be worth trying something different for the first time in years.