Why does current remain same in a series circuit? Current is the rate of flow of charges with time. When passing through each resistance in a series circuit, charges lose energy and "slow" down due to collisions with atoms (metal cations). When time increases, current decreases. I agree number of charges remains same but what about time?
 A: If the current in a series circuit diminished with each resistive device, you'd wind up with a pileup of electrons further down the circuit. If the circuit had a 10A flow at the start of the circuit and a 1A flow at the end, you'd have 10C worth of electrons arriving somewhere and only 1C worth leaving - you'd have 9C worth of electrons piling up somewhere every second! Complete circuits do not spontaneously accumulate charge somewhere in the circuit. 
You can think of it like a garden hose - water entering one end will come out the other end at the same rate, otherwise something is very wrong with your hose.
A: Imagine a scenario. Take a battery having a potential of $V$. The ends of the battery is connected by a wire that is resistance less(an ideal approximation). In that case, the electrons in the wire are offered no 'resistance' and hence, they are free to move from the negative end to the positive end of the battery. From simple application of Ohm's law $$I=\frac{V}{R}$$ we see that the current is not defined, but in the limiting case of $R\rightarrow 0$, $I\rightarrow \infty$. Though the Ohm's law is an approximate law and we need a better treatment of the scenario to understand what is happening, we as of now know that the electrons don't travel through the resistance less wire instantaneously. There are electrons all along in the wire which just 'drift' towards the positive end of the battery without any obstruction and so quite fast(not instantaneously).
Now, if we include a resistor of resistance $R$ in this scenario, the drift of the electrons are no doubt restricted and this restriction of the motion of charges through the resister is indeed what causes the potential drop across the resistor, and the drift through the resistor occurs at a smaller rate, hence the smaller current. 
For two resistors in series, this drift is slowed by both of the resistors(hence lesser current as compared to the single resistor case). However, the drift velocity through the entire wire has to remain the same as electrons can't accumulate anywhere in the wire if there are non regular speeds of their drifts as pointed out by @Nuclear Wang and the OP wrote in the comments. Also, electrons do lose energy due to collisions and that loss appears as heat that reduces the potential in due time and hence the current all in all deceases. But the current can't have various values in such a series circuit.
