Since I'm the one who voted for Infinifactory as their GOTY, allow me to elaborate:
First off, this is pretty much me:
I grew up in a fairly frugal family, so I often find myself waiting until stuff gets cheap before I dive in. I didn't get a PS2 until it was out for about four years, and I didn't get a DS until about a year after the 3DS came out. The games will still be good later on, and I can save a ton of money.
This approach to things is only bolstered by Steam sales and bundles.
So part of the problem is that a lot of my game time in 2015 was taken up by games that didn't come out in 2015. I played through GTA4 and its expansions, and Binding of Isaac: Rebirth was a huge time sink for me.
But as with everything else, there are exceptions. There are just some developers who have earned my trust and I know I love their approach to things. Zachtronics is one of those (Team Meat and Terry Cavanagh are a couple others, and I'm a sucker for anything with a Danny Baronowsky soundtrack, which is why Necrodancer was my #2 for the year).
In case you haven't played any Zachtronics puzzle games (such as SpaceChem, Infinifactory, TIS-100, along with a scattering of web-based games), they can best be described as Sandbox Puzzles. Unlike most other puzzle games, where there's just one or two solutions to any given puzzle, there are a theoretically infinite number of solutions to a Zachtronics puzzle. You're given a set input, told you need a set output, and you have free reign with regards to how you're actually going to do it. Generally speaking, your only limitations are the space of the playing field and a set of failure conditions (for example, in SpaceChem, you can't have any molecules collide with other ones or leave the puzzle area). This allows for a
huge amount of creativity. Each solution is unique, so when you're watching the puzzle make its various widgets properly after what is usually hours of tinkering and refining, it's just so damn satisfying. For example, here's one of my Infinifactory solutions to a later level (made with the in-game gif creator):
I laid out all the blocks in that gif. All you start with is a big empty room (technically, that level is two rooms that you have to get to sync up, but you get the idea), and there are no limitations on part numbers. It's not like you start with a partial factory and just have to fill in the blanks, you're building everything from scratch.