Because of the fitnah of maal (Money) which our beloved last Prophet Hazrat Muhammad (SAW) pointed out 1400 years ago...and also due to kamzoor imaan and being afraid of death... and greed for the position of power...
As soon as a patriotic, honest muslim leader is spotted by the enemy ... he/she will get sidelined or if he/she continued then get assassinated. The astonishing part is that mostly everyone knows this pattern but till now not able to come up with the solution of it.
And then there is another section of Muslims who thinks that wait for Hazrat Imaam Mahdi (AS) till then nothing will work out so why even bother just pray for his early arrival ...
My understanding is that enemy plans and Allah SWT plans much better so Allah SWT probably have selected the right group of people for the revival and it is in the works ... Insha Allah