There are a number of different methods of laying track, and ties-in-ballast is, as you noted, just one method. And all of them have their own strengths and drawbacks.
"Direct fixation" is a catch-all term used to describe rails fixed to a concrete base, and its strengths are that is the most stable in that it is virtually impossible for the track to shift. That makes it well suited for situations where trains will travel through a restricted gauge structure, such as a tunnel. It's also becoming more and more common for high speed lines, as it generally requires less frequent maintenance. Its drawbacks include being the most expensive type of track-laying (when compared to the others), it is far more complicated to calculate and deal with rail-borne vibration, and that drainage is now a far more complicated process.
"Standard" ties-in-ballast is the de-facto standard because it's a known quantity that happens to be very versatile and can be constructed quickly, easily and cheaply. Construction and assembly can be done by machine or by hand. Drainage is seldom an issue, as the ballast should be course enough to allow water through easily. And there are all sorts of different types and size of tie to help deal with any issue that may arise, from high water to termites to tonnage issues to specialwork. Even upgrading the track is generally not a big issue, and there is special machinery that can be used to do so. The downside is that there needs to be an ongoing maintenance regimen in order to keep the track at its desired specification. This isn't just alignment either - while ballast is very good for drainage, over time it can get filled with "fines" (another catch-all used to describe silt, sand, dust - anything that can clog up the ballast and cause it to lose its drainage ability), and thus may need to be cleaned over time.
What the TTC uses is a kind of hybrid between direct fixation and ties-in-ballast - the rails are fixed to large concrete slabs, which are then suspended by rubber in a concrete trough that is poured as part of the tunnelling process. It deals with vibration much better than direct fixation and is far more stable in alignment than ties-in-ballast, but it still has all of the same issues with drainage and cost.
So, why would a metro or LRT prioritize direct fixation over ties-in-ballast? Some of it may come down to gauge - there is not much room between a subway train and the tunnel wall, and thus the keeping of alignment is paramount. As well, maintenance opportunities are scarce, and that may be another reason why direct fixation is chosen.
Automated rail laying and tie changeout machines have existed for the railways for many years. And the base for direct fixation track can be made using a slip-form machine (and is today). But think of it this way - how do you install the rail plates, align everything and install the rail - while the concrete is wet? The answer is that you can't - while the plates and some basic alignment can be done while the concrete sets, the base needs to be cured before you can start to install the rail.
Dan
