TLDR: There was a cornerback they liked a lot available at pick no. 16, so they picked him
The Texans were limited in what they could do in the 2015 draft regarding quarterback, having finished 2014 with a 9-7 record, giving them the 16th pick in the first round. They were one of several teams without a dependable quarterback, but were drafting for instance behind the Buccaneers (1), Titans (2), Redskins (5), Jets (6) and Browns (12), who all could have used a quarterback.
The two highest-rated quarterbacks, Jameis Winston and Marcus Mariota were gone in the first two picks, and trading up into one of the top 2 spots would have been very expensive, if not impossible. No other quarterback was drafted until the third round (Garrett Grayson by the Saints with pick 75) so it would not have crossed their minds to pick any other QB at nr. 16.
But what about other offensive picks? The 20th overall pick Nelson Agholor (Eagles) was still available had they wanted. Obviously the Texans were in the market for a WR, as they picked Jaelen Strong in round 3 and Keith Mumphery in round 5. Why did they prefer to pick CB Kevin Johnson in round 1? This is a question that only the Texan's GM, coach, and scouts could really answer. The answers they gave the press are full of praise for their first-round pick, but of course we knew that already, as otherwise they wouldn't have taken him.
Anyway, here are the quotes of the post first-round media interviews:
GM Rick Smith:
He was by far my favorite corner in the draft. When
you talk about just a complete football player in our opinion, this guy is an intelligent guy, he’s
smart, he’s tough, he can cover, he’s versatile, he plays on, he plays off, he’s got great
instincts, and great play on the ball. In Indy, we really liked him in the room. We thought he
was a good man and just enjoyed his career. He’s another guy that has a career there. He’s a
threeyear starter there.
Coach Bill O'Brien
We’re really happy where it’s at right now. We feel really good about
the corner position, specifically after tonight with the addition of
Kevin (Johnson). We have two really good veteran corners in Johnathan
(Joseph) and Kareem (Jackson) that he can learn behind. He’ll come in
and compete right away.