Absolutely.

The number of elites on the flight affects the number of first-class seats that will be available at T-24, since elites get upgrades to first class (as early as 5 days before departure for 75K, 3 days before departure for Gold, and 2 days before departure for MVP). Some routes are elite-heavy; others are mostly leisure (and can sometimes even depart with empty seats up front).

IAH-SEA[-ANC] is a heavy route for the oil industry, who employs people who travel a great deal between two oil-rich states (TX and AK). As such, I'd imagine that IAH-SEA will have a very large number of elites waiting for first class upgrades, and all of those empty seats you see now will be filled with upgraded elite members a few days before departure.

The good news is that as those elite members get upgraded to first class, their (presumably good) coach seats will open up. The bad news is that most of them will likely be seated in the front half of the coach section, which you don't have access to now, but again there's some good news: starting at 24 hours before departure, you can do Web Check-In for your flight and have access to those "elite" seats in the front half of the coach cabin. So, as described before, do Web Check-In early and often (but once you complete it the first time, only go as far as you need to go to see what seats are open--don't go all the way through the process, as you can only complete the process a limited number of times before it locks you out).

You can also try the free seat alert function at http://www.expertflyer.com, which will send you an email if it finds an open seat matching the criteria you set. (For checks before Web Check-In opens at T-24, be sure to not have it email you if a premium seat opens up, since you won't be able to access premium seats; after T-24, have it check for any aisle seat on the aircraft, as you can choose any seat through the Web Check-In process.)