By a happy accident.
I was, as ever, struggling to stick with an army or get it off the ground, and I found myself doing the mould lines and building a Tyranid Warrior for fun, as antidote to building Space Marines or AdMech that I'd never paint, or endlessly working on scenery for my group's gaming table. It was a weird surprise to actually enjoy painting something for once, so I decided to stick with it and see where it went. After a week or so, it became my seventh ever finished mini:
So, long story short, I decided to stick with them, after I did that test fellow. There's the added fact that me playing Tyranids adds even more variety to our gaming group (which has almost no Space/Chaos Marine players, would you believe), and I've always loved the Tyranids' lore. There's something so completely 40K about the Great Devourer being drawn to the Astronomican: Mankind's salvation and the Emperoer's Light that makes the Imperium possible also being the beacon that draws humanity's doom. (I'm less crazy about the HH/Pharos stuff, so I tend to gloss over that. Ahem.)
I don't know how my army's going to look eventually, but I always adored shooty/magic armies with my High Elves, and practically everything I've ever played since then has been some variation on the same theme. Tyranids have some interesting shooting options, and they have plenty to do in the Psychic Phase, so I'm peachy keen to give that style a go.
Plus, the model range is incredibly strong with the exception of the oldest minis (the two-part heads for 'gants and 'gaunts aren't much fun...), and the Tyranid Warrior kit from 2014 is one of my fave kits of all time. Every single option on the minis looks awesome, and I want to use them all.
As a final personal reason, I don't like playing things that I write about, as it helps in avoiding moronic accusations of author bias, so playing something apart from Space/Chaos Marines helps with that little preference. That's a pleasant side benefit.