Please note:

When the Lincoln Memorial was constructed the names of 48 states were engraved on it. The picture of the Lincoln Memorial on the $5 bill only contains the names of 26 states. These are the 26 states that can be seen on the front side of the Lincoln memorial which is what is pictured on the $5 bill

Ref: http://en.wikipedia.org/wiki/United_States_five-dollar_bill

There are FOUR (4) sides to the Lincoln Memorial, of which only ONE side is shown, so even IF there were 50 state names on the memorial itself, only SOME of the names would be visible on the SINGLE side shown.

John