Each RSS feed usually contains a limited number of posts, so when you subscribe to a new feed in The Old Reader we are only able to fetch the last 20-30 entries. Therefore, without having specific old posts in our database we would not be able to match any previous comments, likes and shares you had in GR to the posts you now have in TOR.
Also, Google Reader only allowed you to subscribe to other people who have Google accounts. We are more flexible and support both Google and Facebook accounts. So, most probably matching previous shares would work poorly, because people who shared that items you had in your Google Reader previously might be authenticated via their Facebook accounts in TOR.
We think that the potential value of this feature is limited (quite a small number of people exported this info from GR), but the effort that would be required to make it work is very substantial. We have so many additional features we would like to bring to The Old Reader; we think that implementing a poorly working import of shares/likes from Google Reader is not worth it.