Look at history
La Cosa Nostra, Triads, Yakuza, Unione Corse - looking at the historic examples of large crime organisations they have a few common characteristics:
- They are crime organisations, not just a bunch of people who commit crimes together.
- They have significant support from a large cultural / national / social group in the wider population.
Organisation is relatively easy to come by, all it takes is a strong leader who can sell to his fellow criminals the idea that planning and organisation make life easier for everyone. (OK, it's a little more complicated than that, but competent leadership can turn a gang into an organisation within a few years.)
The far more difficult task is to gain such widespread support from enough of the population that no one in a large portion of the population will even consider opposing / testifying against the organisation. Short-term intimidation tactics will not suffice - sooner or later some key person will have nothing left to lose - support for the organisation must be ingrained into the population from birth. Historically, there have been two ways these organisations have come into being:
- A resistance organisation opposes a occupying and/or oppressive regime. Resistance organisations are sadly underfunded by the governments they oppose, so typically they fund their activities through criminal enterprises such as prostitution and drug smuggling. The population support them as heroes opposing the invaders / oppressors. Anyone betraying the organisation is a traitor, dealt with harshly and with full support of the population. Eventually either the reason for resistance fades or the passion for it dies but the leaders running the criminal enterprises see no reason to stop making money and the populace tend to keep supporting it out of cultural inertia.
- Disenfranchised / unprotected portion of the population have no recourse to the legitimate authorities when they have grievances. The only source of redress is to appeal to the local crime boss for protection or restitution. Provided the criminal organisation is perceived to be both more just and more feared than the legitimate authorities, the populace will support the criminal organisation wholeheartedly.
There is the answer - in order to become an organisation with nationwide influence, a significant proportion of the population (linked culturally, ethnically or ideologically) must consider the organisation to have greater legitimacy than the actual government. A drug dealing inner city street gang in today's society has no chance of making this leap unless:
- the legitimate government has effectively ceased to govern effectively i.e. the only way to get food, sanitation, education and law enforcement is through the gang; or
- the nation has been invaded and the gang are the centre of the local resistance.
However... there is one situation I can think of that might be superficially plausible. Let us hypothesise that in your world:
- there is a preponderance of scientific evidence for human-caused climate change leading to catastrophic global consequences
- certain governments refuse to accept the evidence and/or take corrective measures
- a majority of the population believes the scientific evidence and that they or their descendants will die if measures are not taken; and
- civil disobedience has been ineffective in convincing governments to take action.
A visionary gang leader who is willing to throw his/her "soldiers" and money into the battle for changing environmental practices could become massively influential. They should work on their public profile to downplay the trafficking of "hard" drugs, but even that will not scare off the more hardline environmental activists. (Drugs destroy some lives, but that is peanuts compared to environmental practices destroying the entire planet.) The gangs provide the money and the firepower and the hardline environmentalists provide technical advice and support (eg cryptographically secure communications, intelligence on targets) that are beyond the education level of the typical gangbanger. The less fanatical environmentalists will not support the gangs directly but will no longer testify against them.
Good luck with your gangs. Hopefully the real world will not need them.