Why? The Theravada chicks are way hotter!
But seriously, the phrasing of this question is loaded with dubious meaning. Why not ask "Good reasons to choose Theravada over Mahayana?"
Now to answer the OP: I was young and had read a number of Mahayana books. But since I wasn't made of money, I needed some inexpensive material on the internet. For the life of me I couldn't figure out how all the Mahayana stuff out there was supposed to fit together into a cohesive piece, as there were so many kinds and so many teachers and the whole scene seemed fractured (1998-99). Then I found AccessToInsight, which had so, so many suttas for free, along with study guides and the teachings of new and old teachers.
So there I was, a neophyte who just found a huge collection of organized material, which was reputed to be closer to the original teachings, all for free. And so I started to read and learn that instead of spending a lot of time and money trying to figure out Mahayana. And it hadn't helped that many Mahayana pages spoke of the need for face to face teachings, empowerments and root teachers, none of which was practicable for a 16 year old in the Canadian wastelands.
Since then I have had read a lot of the suttapitaka and mediated. I have reflected on what I've learned. I've concluded that the Pali canon is the best source for me, and that the teachers who taught or teach from it are the most sensible for me. I've concluded that the Dhamma put forward here is just what I need for ease of mind.