I have owned a 1959 Thunderbird, so similar size and technical, but the Mark II drives ... really better than a Thunderbird or any other Ford (I drove four 50s Ford in my young time).
In my opinion, the interior of the Mark II is almost equivalent to the Silver Cloud interior quality.

Similar "problem" for the last Packard (fifties), they are very low priced and in Europa, nobody want to pay 20k for a Packard Coupe. It is a type or cars very hard to sell in Europa.
A 55-57 Chevrolet Bel Air was a very popular car ... now, it worth 100k (Bel Air Coupe), despite there are still a lot of theses (more than 300.000 were produced for 55-57).

But you can ask the same question for some Rolls-Royce models : why a Silver Shadow worth "only" 25k and a Silver Spirit 15k ?
The best price increase for RR is the Corniche Fixedcoupe model. Price are today 3x the price of 2000-2001.
Not for the Silver Shadow (saloon).

But I don't choose car for the value, I choose with my heart :-)
And I specially love American car 1954-1958 area, hardtop coupe models only.