I'm by no means an expert and I can only talk about my experience with this gear, just to clarify I wasn't in the military, my experience is limited, I was a Cadet in the UK 10ish years ago, I have a small collection of military equipment and gear and I play Airsoft so have some experience with fielding equipment. I see 3 potential causes for getting rid of great coats and moving to newer jacket styles.

1 - technology, materials have become lighter and far better and repelling dirt, water, wind and the elements in general, why carry a large great coat when you can carry a pair of weather resistant/waterproof trousers and a jacket? They fold up small and are lighter to carry and can be pulled out of a pack and put over your battle dress relatively quickly, which follows onto my next point nicely.

2 - weight, great coats are usually made out of older natural materials and methods used are older techniques like sewing and embroidery, the weight alone makes no sense to carry all the weight of one piece of gear when you could easily have a pair of trousers and jacket like mentioned above and still have room and weight spare for ammunition, food, water and other supplies to improve combat effectiveness.

3 - cost, as mentioned above sewing and embroidery, plus natural materials such as wool and cotton all need for the most part large amounts of effort and labour to harvest the material and then manufacture the end product which in this case is a great coat. However you can have all the facilities close together and for the most part speed up production and reduce costs for a modern jacket with modern materials, for example a great coat made from an outer layer of waxed cotton and an inner layer of wool would need a farmer or grower in the case of cotton producing the materials, then harvesting of the material and then transporting to the factory for production, or you could have a factory produce millions of meters of fabrics per hour from raw materials produced close by keeping the cost of transportation down, and then ship the fabric to factory for production, no waiting for sheep's to produce wool or for cotton to grow, which people need to be paid for growing or producing said material, modern materials are quicker to produce, easier to transport and as a result can help bring costs down.

Conclusion, modern jackets are cheaper and easier to manufacture, which result in a cheaper retail price for bulk purchases for militaries, lighter and smaller even when paired with additional equipment like waterproof trousers which means more space for equipment improving combat effectiveness or fighting in said jackets becomes a lot easier on the user when your carrying ammunition, firearms and all the other equipment reducing weight means troops can move easier around the battlefield and modern materials are more weather resistant for the most part than older materials meaning the need for a great jacket isn't as great as it once was.

I personally use to own a Yugo trench coat (not as long as a great coat) and it preformed well, water resistant for the most part but that was helped by a more modern lining as they where produced in the 1970's and 1980's and the coat helped me stay warm. But if I'm going on a long march, long range patrol or going to fight in conditions that requires a jacket I would pick a modern jacket all day long.