Because Brampton isn't central Toronto? I mean, are we really trying to compare that stretch of Main to Eglinton - with two additional interchange stations and especially an urban centre with umpteen times the density? Also Brampton has also put forth a zillion reasons as to why they didn't want it on Main (lack of development potential/ridership growth/other priorities) - only defaulted back to Main but with tunnel as a condition when the province didn't play ball with them. So, why would a ride with no growth potential requires a tunnel - and only after a second round of consideration - is beyond me.
Never say in perpetuity of course - but forseeable future, in the absence of a change in mind? You bet. Now if you are comparing it against say the Eglinton West burial plan and this, perhaps there is a good point - and more about the irrationality of the latter than the defensibly of the former.
AoD