Why?, I'll say there's 3 basic reasons for me, and 2 are financial reasons for SOMETIMES opting for 30 minutes:
1) New girls are a financial risk. I don't want to commit $200 for a girl who might have bad teeth, horrible breath, bad smell "down there". If it's a good session, I'll be back for the one hour Round Two.
2) Do the math. Using the common denominator of $1,400, this amount of cash gives me 10 half-hour sessions compared to only 7 one-hour sessions. That's an increase of over 40 %! Now you know where the phrase, "More bang for your buck" comes from.
3) Time. Having nothing to do with available funds, but just available minutes. Many times 30 minutes is all I have, ... probably don't even have that, but I can squeeze in a half hour.
Of course, if it's an ATF candidate, I'll wait till I have a full hour to spend with her, because I know my money isn't at risk and I'm guaranteed a good time.