Customs authorities had adopted the practice of screening international cargo, as it arrived in the country, compared to having the cargo pre-screened, before it leaves the country from where it is being exported.  
However, Technological shifts and changing paradigms, have led to the emergence of issues that pose a threat to national security. As a result, customs authorities over the world are increasingly shifting towards pre-screening exports.

It is a reasonable to question, why such a fundamental matter was brought to attention after such a long period of time?

The International Maritime Organization (IMO) and International Civil Aviation Organization (ICAO) have been aware of the risk in security associated with transporting cargo and passengers. And have been focusing on security measures to be improved for over decades now.

So why customs authorities have not given this matter the attention, that it required till late?

This can be found in examining the policies regarding crossing borders at a national and international level. The primary aims of transport authorities and Customs differ much. Security and safety are primary priorities for authorities of the transport sector. Whereas the primary concern of Customs are ensuring compliance with international and national trade laws and the collection of revenue.

Furthermore, It is possible for the greater part of regulatory responsibilities to be dealt with by reliance. These will be dealt on conventional methods of compliance management. Therefore, Risks to security need to be identified and addressed through prevention at the origin point. Rather than detection at the arrival point. Hence, The priorities of a government’s administrative machinery are dictated by political forces. Furthermore, it is only recently that national security has been considered a priority. That needed to be addressed by customs authorities.

What is the current practice?

Customs authorities have up till now, followed an “interventionist” approach, i.e. intervention being necessary to fulfill regulatory requirements. However, The latter years of the 20th century have seen this approach change. Paradigm shifts in the way international trade was conducted, saw this approach change.
There was increasing pressure on governments. From global businesses and trading community to reduce the intervention of governments in business transactions. However, this pressure was further compounded by the emergence of the global marketplace and advancements in technology. This have led to transforming global trade.

Furthermore, Pre-Export Screening gained impetus after the terrorist attacks in September 2001. The World Customs Organization and the ICAO have mutually agreed that a global supply chain, can only be secure if there are adequate security measures enforced at the point of origin.
Several other organizations of Pre-Export Screening and examinations of security strength have resulted in measures, being taken to enforce pre export screening for sea transport. However these measures need to pick up pace as far as air transport goes.