Yes, the ruggedness is the main priority. Once a piece of hardware is certified and flight-tested, you have so much invested in the computer design that you don't want to just throw away the design because there are faster chips for sale.
And there's the question of whether the extra processing power is beneficial for the task at hand. Why pay more for extra processing power that isn't used anyway? There's likely a finer degree of control and timing now, but it's not like reentry physics has gotten more complicated in the past 12 years.