Why is it difficult to develop wide-bandwidth analog-to-digital converters?
all electronics have limits at the high-frequencies due to physical realities.  monolithic integrated circuits have a sorta problem that every internal device (every BJT, FET, MOSFET) is connected to a common substrate via a reversed-biased diode.  while reversed-biased diode junctions are normally understood to be no connection, there is a tiny amount of leakage.  in addition there are little walls of charge hanging out on both sides of the reversed-biased junction which are very similar to a charged capacitor.  so it's like a little femto-farad capacitor is connecting every node to a common substrate.  at sufficiently high frequencies, even that femto-farad capacitor will begin to conduct and to short circuit.