A few reasons.
1. Nebraska. The amount of tanks over 55 is slim. 75's aren't all that common and it just gets less and less the more the gallonage goes up. I'm still open to a 125 if i find a good deal on it before i find a good deal on the 75, but whichever i find a good deal that circumstances work out with I'm taking it.
2. Transport. I only have a small car, a 75 probably won't even fit in it, so I'm forced to either try to pay delivery or if its not all that far away i can borrow a pickup.
3. Price. There has been 1 decent deal on a 125 in the year i have been looking for a larger tank. I tried to get it, but there was no way i could pick it up. It was $50 for a 125, it needed to be cleaned out well and the center brace needed to be replaced, but i would have took it in a heartbeat had i been able to. The cheapest 125 tank besides that has been in the ballpark of $400. 75 gallons usually don't even go for very cheap, but finding a good deal on them is slightly easier.