Maybe because tech firms are starting to cluster downtown. Didn't google move into a new office downtown last year or something?
http://m.phys.org/news/2012-08-tech-firms-shift-silicon-valley.html
This is true for smaller or medium sized tech companies, but usually not for larger campuses like IBM, Google, Microsoft or Facebook. (Many of these have satellite offices in urban areas however, the biggest probably being Google NYC)
One big factor in choosing whether you want a suburban or urban office location is who you want to attract and retain as employees. If you're after the younger, single crowd, they would most likely value a downtown office within walking distance of their condo at King & Spadina or Liberty Village. A suburban office park in Markham would more likely attract those who want or need a larger house for less money per square foot, or those who appreciate the quality of Chinese restaurants in the area, or those who enjoy the suburban lifestyle.
Office location is a big factor when recruiting talent. Imagine you're trying to recruit a recent graduate. If you're office is say at King & Bathurst downtown Toronto, you can sell them on the exciting lifestyle, good food, bars and coffee, and urban environment (and lots of other young single people around). On the other hand, they might be considering an offer (of much more money) from Google in California. In that case, they either have to live in a very suburban area (with beautiful natural surroundings), or live in SF and take an hour bus each way every day.
That's why smaller companies can use office location to their advantage when recruiting.
I don't think IBM would simply move everyone from Markham to Downtown, there would be too many employees that would hate the move. Creating an additional office is plausible though.