There are several points here. I grew up on Balne Lane and spent most of my time on those playing fields which we used to call "The Tips". It may sound like a peculiar name until you realise that that is just what they were at one time - the council rubbish dump!. Once they had got to a suitable level some soil was dumped on top and playing fields created. This was back in the late 40's, early 50's. They were then OK for a long time with football and rugby pitches, a cricket pitch (tarmac!) and a building for changing rooms. Eventually it fell into disrepair due to lack of maintenance. Many, many years later I worked at Wakefield College and I was asked if I knew anything about the playing fields. I think that the college owned them at that point and was looking at selling them off as building land to get some money. However there was this clause about community use. The problem for the college was they couldn't be used as sports fields due to broken glass, tin cans and all sorts of other stuff from the council tip days being exposed by erosion etc. It would have been too expensive to dig it out to a sufficient depth, make sure there weren't any nasties like asbestos in it and then deposit an enormous amount of topsoil on it. In the end nothing happened and the site is as it is at the moment - a right headache for whoever owns it. I reckon that the cost of "rehabilitating" the land would be too much for anyone given the covenant on the land.
The next point is access. The streets around the site are all very narrow with a limited number of approaches. At Belle Vue you can approach from 4 different directions with ease. With Balne Lane playing fields there is only Balne Lane itself which is not very wide and has on street parking. There is no road to the fields themselves, only a small lane with a bridge that could not take the weight (literally and figuratively) of the traffic. It is wide enough for one car width and would probably collapse under weight of a decent 4x4!
Linked to that point is the access from town via Balne Lane. There is a bridge under the main Westgate - Leeds main line. This isn't wide enough for 2 cars to pass in compfort and give way markings have been put on the down side to avoid problems. Anyone who goes that way at rush hour can tell you what a problem it is. It could not carry all the match day traffic from the north of the city. From other direction traffic would have to go through Flanshaw Estate, again with narrow roads. There is no way that any planning officer could approve such a plan without enormous road changes with (probably) astronomic expense. WMDC doesn't have that kind of money.
I could go on but it's becoming an essay rather than a post so I'll shut up now except to say it seems a load of b******s!