Why Is The 2010-S Proof Lincoln Shield Cent So Expensive?

The main reason why the 2009 (Clad) Proof Sets were broken up in such
huge numbers was the Set's Copper Cent lens.

The Cents were not announced to be available as a standalone Set at
the time of the Clad Proof Set's release, so for a few months in 2009
loose Cent lenses alone were selling for up to $18 on ebay (and
around $15 even in local coin shops).

So, from a $29.95 Set, you could sell one of its four lenses for $18.
If you sold the remaining three lenses for more than $11.95 total, you
made a profit. Two of the remaining lenses in the broken-up Set were the
Quarter Set and the Presidential dollar Set, which were sold by the
Mint as standalone Sets for $14.95 each (albeit with a box and COA).

You could then sell the loose Dollar and Quarter Sets for $8-10 each,
then maybe $5-6 for the leftover "Sac/Half/Dime/Nickel" lens.

By the time the 2009 Silver Proof Set was released, the Mint had announced
that a $7.95 Proof Cent-only Set would be released (which sold out quickly).

The Proof Shield Cent is not available as a standalone product.
(from the Mint)

The experiences of 2009 and the current high prices of broken-out
Shield Cents suggest there's a fairly large market that want to buy
Proof Cents, but no other Proof coins.