Short answer-yes, and for several reasons. The popularisation of the sinsemilla technique (growing unfertilised females which are seedless) greatly increased the potency of cannabis, as the plant produces many more flowers (buds) and does not "waste" resources producing seed.
Selective breeding has also played a role. Although cannabis has been selectively bred for centuries, modern scientific understanding, and the capacity for people to cross strains from different geographical areas, have allowed the creation of varieties that have many desirable attributes, including higher potency. When you say "genetically modified", if you mean the kind of high-tech splicing in a lab that Monsanto do, no, this is not done with cannabis. However, the kind of "genetic modification" that people have practiced since before history, that has produced (for instance) different breeds of dog, has played a part.
However, much of the increase in potency is seen with indoor-grown weed, and is due to improved growing conditions (in terms of limiting factors). Indoor growing is very modern compared to the history of cannabis as a whole, and high-powered lights and fertilisers have greatly increased the strength of the average street cannabis, as production has moved from extensive outdoor farms in foreign countries to domestic, indoor production. This is largely due to the ever-rising likelihood of seizures, as well as the soaring price available for cannabis.
Even a landrace variety of cannabis (one that is not intentionally improved by humans) will produce relatively potent bud if grown with modern techniques indoors). So, in answer to your question, cannabis has always had the potential to be as potent as it is now, but it has never before had the opportunity to realise that potential. The weed being smoked today is not some mutant superskunk, the difference between it and the schwag you are used to is one of degree, not of kind. The fact that you cannot find it is more due to economics than it is to technology.
Hopefully that all makes sense, feel free to ask if something is unclear.