[Update: I accidentally left 3 runs out of the 2 possession game numbers, counted drive #4 as a one possession lead drive, when it was 2 and counted Arian Foster's single reception as a run in drive 2. The numbers should all be correct now, I double checked all the totals]
The Colts defense is built with the Colts O in mind. An emphasis on speed and quickness over size and strength combined with safeties sitting deep the vast majority of the game isn't a philosophy embraced with defending 40 run plays a game in mind. By philosophy, scheme and roster construction the Colts D is pass defense first. The reason this has worked to the tune of 8 straight playoff berths and 10+ win seasons, 7 straight 12+ win seasons, 2 AFC titles and a Super Bowl victory is because the Colts have an offense that regularly forces the oppositions hand in their playcalling.
The Colts offense is one of, if not the most important component of their defense. The offense's job is to shift the opponents run-pass balance, forcing the opponent to battle through the vicious teeth of the D and ignore it's vulnerable underbelly.
Excluding the final kneedowns the Texans had 9 drives. Here's how they ran on each:
(Success Rate is a football Outsiders stat, a play is considered a success if it gains at least 40% of the yardage to go on 1st down, 60% of the yardage to go on 2nd or 100% on 3rd or 4th down. When leading in the 4th quarter the benchmarks are reduced to 30%/50%/100%)
Drive 1 (tied): 3 runs, 11 yards, 66% success rate, 3.67 YPC
Drive 2 (up 3): 0 runs
Drive 3 (up 6): 4 runs, 14 yards, 75% success rate, 3.5 YPC
Drive 4 (up 13): 2 runs, 17 yards, 100% success rate, 8.5 YPC
Drive 5 (up 6): 1 run, -2 yards, 0% success rate, -2 YPC
Drive 6 (up 3): 13 runs, 60 yards, 77% success rate, 4.6 YPC
Drive 7 (up 10): 5 runs, 28 yards, 80% success rate, 5.6 YPC
Drive 8 (up 10): 4 runs, 91 yards, 100% success rate, 22.8 YPC
Drive 9 (up 10): 7 runs, 41 yards, 86% success rate, 5.86 YPC
Looks pretty noisy, but they come together in major trends when groups by quarter and by the Texans lead.
1st Quarter: 6 runs, 24 yards, 83% success rate, 4 YPC
2nd Quarter: 4 runs, 18 yards, 50% success rate, 4.50 YPC
3rd Quarter: 16 runs, 82 yards, 81% success rate 5.13 YPC
4th Quarter: 13 runs, 136 yards, 85% success rate 10.5 YPC
Tied: 3 runs, 11 yards, 66% success rate, 3.67 YPC
1 possession lead: 18 runs, 72 yards, 72% success rate, 4 YPC
2 possession lead: 18 runs, 177 yards, 89% success rate, 9.8 YPC
The Colts run defense actually held while the game was close, but utterly failed when the Texans widened the gap. The Texans run splits by lead totally changed my view of the loss. The run D didn't put Indy out of the game at all, it just killed any chance of a comeback. The Colts MUST stay tight with their opponent to keep the defense from being pounded into oblivion by the run.