Because we would benefit whether we got promoted or not as he'd be our player. Let's say we get him on loan and he makes a great impact like MGW and McAtee but we don't get promoted. Villa might then sell him for £10-£15mil to another top flight club. If we sign him permanently (for a fee lower than that now), then if we don't go up and clubs are interested then we sell him for £10-£15mil then we get all the benefits.
I'm not against loans but how much money did Wolves, Man City and Man Utd make from the sales of Henderson, MGW, Doyle and soon to to be McAtee thanks to us, not to mention the loan fees paid to them.
Loan to buy deals very rarely end up that way. There's rarely an obligatory if you loan them then you must buy them. They're normally a deal of if you want to sign them after loaning them then you get first option and we'll agree a deal now, regardless of how well they play on loan, or if you get promoted and they play x number of games then that will trigger the afore mentioned deal.
I can think of very few, if any, loans where we went on to buy them that season or at the end of a season. Kabba and Webber are the only ones that immediately spring to mind.