WWE worried about putting Undertaker against Brock due to Brock's aggressive style...
Saw this article on it, apparently, there is concern about putting Undertaker in the ring with Lesnar because Undertaker has not been in great shape physically lately, and Brock Lesnar's stiff/aggressive style is a concern for Undertaker's physical condition:
The first match being talked about is between Lesnar and CM Punk, and the other would see Lesnar face The Undertaker. The Observer said Lesnar versus Undertaker would potentially be a huge draw, but it all depends on Undertaker's physical condition. A reference was made to Taker's on-screen standoff with Lesnar at UFC 121 being incorporated into the feud, but Undertaker is said to be in bad shape and a match between the two might not work well because of Lesnar's physical style.