Because 5x5 w/ 225 = 25 reps with 225. That totals 5625 lbs moved. 1x8-9 w/225 = 1800-2025 total lbs. moved. Additionally, you can exert more force when doing more sets of fewer reps while not going to failure than by doing one or two sets to failure. Think about it. Your first few reps of your sets, you are exerting much more than 225 lbs-- the weight goes up much faster on the first few reps than it does on the last few. Therefore, in addition to moving over twice the total weight per workout (which will undoubtedly lead to greater muscle growth), you are exerting more force per rep, which increases muscle tension and power, which also leads to greater muscle growth.
If that is too complex for you to comprehend, just look at it this way-- why would 9 reps with 225 per workout lead to greater growth or strength than 25 reps with 225 per workout? Doesn't it make sense that someone who lifts nearly 2.5 times the amount of weight (5000+lbs) would be stronger and bigger than the other (2000lbs)?