Have nuclear tipped SLCMs been considered for example? If you want to talk economics, how many of those could you get for the same money?
If you could get say ten times as many, is that a more viable/versatile option even after you factor in it's presumed greater vulnerability to air defences?
For the deterrent to be meaningful, you need to pretty much guarantee some of your warheads will get through, which is effectively impossible when using cruise missiles against a well-defended target.
ICBM warheads are very difficult and expensive to intercept; for a flavour of some of the measures used to ensure a target is reached, have a look at the Wikipedia page for Chevaline.
As for using submarines - any land-based system puts a premium on a first strike, because you have to use it before you lose it. Not only do you want a submarine-based system, so that you can threaten to use it in retaliation; you also want your nuclear-armed adversary to have a submarine-based system. If they feel their system is invulnerable, they won't feel pressured into a first strike.