Heres why, and ive explained this earlier. The median design was done in 2008ish as a first draft, and there were plans to decide to maybe tunnel or elevate or put to the south side later.
However, in 2010 Rob Ford became mayor and demanded that the Eglinton line be entirely underground. He did this on the provision that ammendments were to be made to this Leslie section, and potentially other areas.
In order to stop this, city council voted to stop the changes, and go to the original design of in-median.
This meant that while Rob Ford was mayor, no changes to the LRT EA or plans could happen because it would mean re-opening the debate to have the line buried or changed drastically.
So we have Leslie as is because in order to get the project to go ahead on time and not allow for a re-opening of the debate of tunnelled vs above ground, the only option was to build the line unaltered.