1. Low publicity.
2. Asked too many questions (i.e. what does it mean to be an entrepreneur?)
3. Deadline too soon.
They should've kept the deadline for applications at April 12th since by that time, all YC applicants will know if they have been accepted/rejected. The rejected ones could then apply at TechStars. This is not necessarily a bad thing since YC only picks 15 teams, while leaving a big pool of other great startup ideas and founders behind.
I have had phone conversations with numerous entrepreneurs on news.yc, and almost all of them plan on applying to TechStars while keeping YC as their first option.
TechStars could have, for marketing purposes, used aikido strategy and turn Y-Combinator's strength into weakness: since TechStars is new, they could've claimed that they place greater focus on the startups they fund as a way to prove their model superior to YC. A lot of people would've bought into that argument.
Having competition actually validates Y-Combinator's business model. While two similar programs may not be good for either parties, it is definitely good for the prospective founders.