Should not "Dlaczego ten ptak frunie tak wolno?" be acceptable.
"fruwać" is definitely a less common word, but we added it somewhere else, so added here as well.
Why not lata?
"lata" would suggest that the bird is generally slow, for which you'd use "this bird flies".
So "lecieć" is present continuous whereas "latać" is present simple, just like "nosić" and nieść" respectively?
Plus "latać" is also multidirectional, so it can work for 'flying around' in Present Continuous.
Is not "D. leci ten ptak t. w." acceptable word order too?
No, that doesn't work.