Good question.
It probably has to do with how the address is formatted. The iPad looks at the text, and if it thinks it looks like an address then it underlines it and offers it up as a link to maps. It does this in Mail and some of the other built in apps as well as Calendar.
I have no idea what the parameters are, and obviously they will be different depending on what country you are in. All I can suggest it to take a good look at the ones that work, and the ones that don't work, and try to make the second look more like the former.
You can always try tap and holding the address info to select and copy it; then open the Map app and paste it into the search.
Good luck.