I can see how an overworked engineer, with another two aircraft to turn around might notice the covers and think "I must get some steps to take those off". But then, the fueler comes over to have the chit signed and someone else comes up to him and says a cargo roller has jammed in the forward hold. On his way to the cockpit to write up the fuel in the tech log, someone says the rear sink is blocked or the IFE is not working etc, etc.
I can see that the first item - the covers - could get forgotten. But why did the pilots not notice during their walk around? Do they really need a streamer to see if a cover is on a pitot probe? Why not simply just look at each probe - I always do, (as well as each landing gear pin hole or strut to check no lock pins have been left in).
I would hope that engineers have a checklist to make sure everything gets done and not missed on the turnaround, and also it seems that pilots need a more comprehensive checklist than the present "Gear pins and covers?",
e.g:
Engine cowls? Closed and locked
Gear pins? Removed
Engine blanks? Removed
Pitot covers? Removed
Static covers? Removed
AoA vanes? Clear
Fuel panel? Closed
Our industry used to pride itself on learning from mistakes. Let's learn from this one. Ropes attached to the pitot covers so they can be removed from ground level without having to go and find steps. Checklists for engineers. Instruction for pilots on how to check the aircraft on a walk-around, and I don't mean a series of written words in an FCOM; I mean someone actually physically showing the pilots where the ground lock pin holes are on the undercarriage struts. Where the pitot probes are. What to look for and where everything is.
Training, training, training.