Why are the airplanes losing speed?

[dexter]

Flying on an airplane is the fastest option for the worldwide travel. And, with the advent of jet engines, the airliners became much faster, giving travelers unprecedented opportunities. However, over the past decades, airliners have not just not increased flight speed - they often fly at speed lower than their predecessors. This strange fact can raise the question: why, despite all the progress, do we fly so slowly? Fuel? Money? Regulations? Let's find out!

 

[Arsalan]

Flying on an airplane is the fastest option for the worldwide travel. And, with the advent of jet engines, the airliners became much faster, giving travelers unprecedented opportunities. However, over the past decades, airliners have not just not increased flight speed - they often fly at speed lower than their predecessors. This strange fact can raise the question: why, despite all the progress, do we fly so slowly? Fuel? Money? Regulations? Let's find out!

You answered you question with your last line indirectly.
Its all three. Fuel, Money (which are co-related) and regulations.