Because, if they miss double 15 and get a single 15 instead, it leaves a two dart finish where going for treble 10 and getting single 10 still leaves a possible one dart finish?
Why do players generally go for a treble to set up a finish and not a double? Because if you miss the double to the outside you score nothing. Miss the treble and you'll get the single points. However, there's a growing trend amongst some pros to go for two double tops when they need 80.
Why is it "not the done thing" to go for a bullseye to finish? Never heard that it wasn't and I've played darts for decades. Good players rarely go for a bull finish if they have more than one dart in their hand because it's a much smaller target than a double is and if you miss it chances are you won't get a second dart at a double anyway because you'll either be left with an odd number or an even one higher than 40. That's why typically stepping up to the board with 50 to get they don't throw at bull because the odds don't favour it as a finish. However some do and if they do I don't see why it would cause offence.
most dall the outs are with or within there 3 dart out.art player go for treble 10 when they need 70 out.
70.if they hit the treble 10 thats a score of 30 then they need double top for out.
BUT
if the go for the treble 10 and hit a single 10
then they go for a single 20 to leave double top.