Why don't they farm this herb in a massive amounts and give it to every citizen, so that they all could have super powers?
From a philosophical point of view, the world is perfectly balanced between good and evil. It is all about collateral damage. In everyday life, for normal human disagreements, the collateral damage will be less but for a super-powered human disagreements, the collateral damage will be much much more.
The best analogy from real life I could think of is to replace Wakanda with America and Heart-shaped herb with guns. We can see the results of giving everyone a (super)power almost everyday in the news. Lots of innocent bystanders are killed.
With great power comes great responsibility and whoever shall be worthy should possess that power.
There could be multiple reasons to why the Wakandans don't feed the Heart Shaped herb to everybody.
Wakanda is essentially a monarchy where the king rules with the help of his council. It will not be in the ruler's interest to make a herb that bestows its consumer superhuman strength and reflexes available to anybody and everybody. A rogue citizen who wants to overthrow the king would indeed benefit from the Heart Shaped herb. For instance, what do you think would have happened if M'Baku had access to the herb?
Moreover, this would cause the Wakandan society to break down as everybody would become a physically powerful (read superhuman) being. And as far as giving it to their military goes, Wakanda hasn't really been a conquesting country. They rely on secrecy and the force-field around their country to keep out unwanted visitors.
It is mentioned in the movie that the herb is a gift from Bast - the Panther Goddess. It is meant for the worthiest warrior (the King). Giving it to everyone is tantamount to disrespecting and angering the goddess.
Not everyone is worthy of consuming the herb. An unworthy recipient will have violent reaction to the herb which can be fatal. Although this wasn't really expounded upon in the movie, it has happened in the comics universe.
At present, the Wakandans grow the Heart Shaped herb in a small garden of sorts since its consumption is restricted to a single individual. If the number of consumers grow exponentially to the entire population of the country, they will have to start farming it on a much larger scale. It would require a large work force to tend to the crops (remember how many women were in the garden tending to such a small bounty). This would also bring forth accountability issues as some of this crop could end up in foreign hands becoming a different kind of threat altogether.
At the end, the reason is a sum of these factors. Ultimately the Wakandan's philosophy, social structure and their religious belief regulates their use of the heart-shaped herb.