Not sure if this fits or not, but for a variety of reasons (partly due to lack of interest, complete secrecy on part of the organisation and the translation involved) there are very few English language books on this specific subject.

Even in the original Italian, the nature of 'ndrangheta is so disparate and disjointed that piecing together any coherent reasons for it being perceived to grow so rapidly is difficult. Court transcripts make it extremely difficult to follow any sort of overall thread between the various (rare) people they actually manage to catch and successfully prosecute.

If English language is your needs then I can only recommend two books by John dickie, "Italian brotherhood's" and "mafia Republic" which look at the rise and fall of the various Italian organised crime factions. They are very good historical books, not just "gangster books" if that makes sense.

Some very high level reasons include the comparative reduction in visibility on the part of Cosa nostra following the assassinations of falcone and borsellino in 1992. Following this were some mainland terrorist attacks also, but in 1993 the then de facto capo dei capi Totò Riina was captured. The subsequent boss was Bernardo Provenzano who took Cosa nostra underground to take the heat off. There is much speculation about deals and pacts made with the state about this, with few verified accounts.

This meant to some degree that the 'ndrangheta mopped up some of the trade, especially in international drug trafficking.

However it's also important to remember that it wasn't until the 80s and the maxi trial in Sicily that there was widespread and legal acceptance that there was organised crime groups in Italy. But it could be argued that of course there was world wide knowledge of the existence of the "Sicilian mafia".

Yet in this case it wasn't until as late as duisberg massacre in 2007 that many people outside of Italy had ever heard of 'ndrangheta.

It is still less of one overall coherent group ala cosa nostra, and this has led to it becoming stronger over time as many individual powerful worldwide families are all attributed to the same group but without any sort of central leadership.

Tl/Dr - mixture of extreme secrecy, growth over time and Cosa nostra reducing levels of activity in second half of the 90s.