But the concept itself failed.
Let's go back to late 2009 to read the dev blog introducing Strategic Cruisers to the players of Eve:
The elation of happening upon a unique fit, some fresh and clever setup, has always been a very appealing aspect of EVE. One of our primary goals for the Tech 3 ships was to bring this customization to a whole new level. The new ships are a fundamental departure from the tried and tested, offering such unparalleled customization that discussing them on the same terms as previous ships belies their true extent - we're talking full-fledged, independent systems within the system. A little bit scary, really. They are called Strategic Cruisers, where "strategic" refers to the internal scalability and dynamic nature of the setup options- not a battlefield role like coordinators or command ships. Of course, they can align beautifully with virtually any fleet on any mission, but just as well prowl as lone wolves.
Subsystems not only change the ship's attributes, slot layout and bonuses, but the geometrical shape and appearance as well. As stated, each vessel consists of 5 subsystems and a hull. Once the ship has been assembled, players are free to switch out the subsystems completely at will, given they are docked at a station.
Emphasis mine. The concept is simple: you can pick what role / bonuses / slot layout your ship has AND you can change it whenever you want. The first part is part of the reason the ships are so ubiquitous but the second part has pretty much failed miserably. Most of the time, you use a tool like EFT or Pyfa to determine what setup you want for your Strat Cruiser including the 5 subsystems you want, and then you buy that setup, put it together, and most likely never change it again.
If you want a Strategic Cruiser for a different role, e.g. a probing cloaky ship instead or your sanctum running missile spammer, you are more likely to simply buy an entire second ship rather than just the mods to switch your current ship. My hanger, for example, has three Strategic cruisers in it: one for PvE, one for cloaky probing, and one for pure gank PvP.
What is the reason for the failure of the ship class to live up to its potential? There are several:
1) Rigs. When you put three rigs on a ship, they are usually to compliment its main role or main method of tanking (or sometimes both). However, changing the subsystems of a strat cruiser can vastly change its role and/or optimal tanking method, most likely making the installed rigs sub-optimal or useless. Since you can't just remove rigs, only destroy them, it makes players committed to a single setup once the rigs are installed.
2) In Game, Setting Up Ships is Tedious. While some big improvements to fitting a ship in game have been made over the years, the fact of the matter is that it is downright tedious and intensive to explore ship setups and tweak it to how you want. There is no easy way to compare stats of two ship setups, removed mods are assembled so don't stack and litter up your hanger, your hanger modules are not in a nice tree to browse like they are in the market, and unless your hanger is well stocked or you are in a market hub sometimes you just don't have the modules or charges you need for a quick setup change.While out of game tools help a lot with some of these deficiencies, the fact of the matter is that its easier to setup a new ship with brand new purchased items than it is to swap out subsystems and modules of an existing setup unless you don't want to use that setup ever again.
3) Too Limited. Many of the scenarios where having the ability to change the capabilities of your current ship on the fly would be useful are the same scenarios where you do not have the ability of swapping subsystems and modules: deep in enemy space, wormholes, during a long roam, etc, any time you are far from your home base. Even if you have the subsystems and modules available, since you have to be docked at a station you still can't use a POS hanger or capital ship maintenance bay to to the switch anyways. So no jumping from Sleeper-farming PvE ship to hostile-fighting PvP ship in wormholes and no switching from forward-scouting recon ship to damage-dealing tackle ship during an offensive in null sec.
* * * * *
So what is the answer to make Strategic Cruisers live up to their full potential? Easy; address these three problems in some manner and you will go a long ways to getting there. Introduce a method for removing rigs (i.e. only in station and for a cost), introduce more ship-building-theory tools and module hanger organization tools into the game client, and allow swapping of subsystems outside of station environments.