Stuck Front Shock in Strut

[Steve S]

I am replacing my front strut cartridges (aka shocks), and have the tool 
to take the cap out.  The pass side went without a hitch.  But the 
driver's side shock cartridge wont come out of the strut housing.  I've 
tried tapping up with a hammer on the end of the shock, grabbing the lip 
of the shock from at the top of the strut with a channel lock, but 
nothing budges that shock.  For now I've put some penetrating oil on the 
thing and will let it sit over nite.

The nut on the strut wasn't rusty, and I can't come up with any good 
theories as to how this got so tight in there.

Any BTDTs, or other ideas as to what's happened here and how to remedy it?

TIA