I think that TZ isn't as accessible for most people and the fact it's still in the top 5 is a testimony to it's design. Even after a year I still find new things to try, or experience - for example right now I just try and focus on powerball - getting it out/putting it back etc to work on that shot. It's like a little reward to play powerball once it's loose.
You'll read many reviews stating that it's slow, and I believe that's in how it's set up. Mine plays super slick but I keep it waxed on the regular and closer to 7 deg pitch, it's never slow (try nailing the orbit without magnet intervention)
Yes the game has a ton of modes and that can slow things down a bit, but TZ is a great standalone game or companion to the other top listed games which are amazing, but in the end have accessibility over depth. I only have space for 4 machines, any time I have people over they gravitate towards MM or MB, and Shadow and TZ get a few plays before people get chewed up and spit out - I like having that kind of mix.