Can someone please explain, intuitively ...
First of all:
The definition $C = \frac{Q}{U}$ is a random definition made in the 18th or 19th century.
Back then, they could have defined the word $\text {"capacitance"} = C^* = \frac{U}{Q}$ instead. (Indeed this value is named "elastance" today. Thanks Alfred Centauri for your comment.)
If they had done this, the "capacitance" (which would be $\frac {U}{Q}$) of capacitors in series would increase and not decrease!
For this reason I doubt that it is possible to explain the phenomenon "intuitively" without at least referring to the formula $C=\frac {Q}{U}$.
I'd also like to give a generic answer that also applies to "real" capacitors not having plates...
... why the equivalent capacitance of capacitors in series is less than the any individual capacitor's capacitance?
First you should remember what you are talking about if you talk about the "capacitance of a series connection":
You are talking about the voltage measured over both ends of the series connection and the charge that was flowing into one end of the series connection.
You are not talking about the voltages measured inside the series connection and/or charges somewhere inside the series connection.
If some electrons are flowing into one end of a capacitor or one end of the series connection, the same amount of electrons will flow out of the capacitor or series connection at the other end. This amount of electrons is the "charge of the capacitor" $Q$.
In the series connection the electrons that flow out of the first capacitor will flow into the second capacitor. This means that if some charge $Q$ flows into one end of the series connection, all capacitors will be charged with a charge of $Q$.
Because we defined the charge that was flowing into one end of the series connection as "charge of the series connection", the "charge of the series connection" is only $Q$, not $N\times Q$ if there are $N$ capacitors in series that all have a charge of $Q$.
On the other hand the voltage $U$ describes the energy that is needed to transport an electron from one point in a circuit to another one. To transport some electron from one end of the series connection to the other end, we need the energy to transport the electron from one end of the first capacitor to the other end of the first capacitor. And we need energy to transport it from one end of the second capacitor to the other end of it. This means that the voltage over the series connection is the sum of the voltages of the capacitors.
Now we are at the point where we need the formula $C=\frac{Q}{U}$ because things would be completely different if the "capacitance" was defined as $C^{*}=\frac{U}{Q}$:
Because the voltages sum up but the charge of the series connection is equal to the charge of each single capacitor, the capacitance of the series is:
$\displaystyle{C = \frac{Q}{\sum U_\text{capacitor}}}$
This means that the numerator of the fraction $\frac{Q}{U}$ is the same for the single capacitor and the series connection, but the denominator is larger in the series connection.