Because that is all you can do,leaded windows on a church have to look aged,not dirty or sparkling clean,its called keeping in look with the building,lead and stained glass has a patena(a certain look) to it,that must be kept,you cant have churches 100s years old with brand new looking lead and windows,they just dont go......apparantly.That is why the Church Council wont let you clean with anything else....Im not saying the Vicar/Pastor/Priest wont,He/She might just not know...