The answer is simple: it is because Bulgaria has adopted the railway system that is normal throughout most of continental Europe. In Europe it is usual for the carriages to have steps leading up to the doors, therefore the platforms have been built to fit the trains. Britain is an island with historically no rail connections with the rest of Europe. The early British engineers had the luxury of being able to do things differently. They decided to have high carriage doorsteps and high platforms. Having designed a certain system you then have to stick to it, to avoid huge expense and uproar.
British carriages are also narrower in width. It is impossible for normal continental carriages to travel on the older British standard rail routes because they would not pass under the bridges or fit though any of the tunnels. Spain and Portugal also have a different rail system to the rest of Europe. French trains have to stop at the border and the passengers have to change to a Spanish train; but it is only the wheel gauge that is different, I think that even south of the Pyrenees there are low platforms and carriage steps, so Bulgaria is certainly not alone.
It is not the Bulgarians that are out of line, it is the Brits. Driving on the left hand side of the road also comes to mind, and then there is the electrical wiring system, with “ring main” circuits and low rated fuses in each of the plugs. The best system in the world - but no other country will adopt it and why should they? It would simply be too expensive to change.
There is one advantage that I can think of in having low carriage entrances and that is that the train can can stop anywhere on the line away from a station and people can disembark without having to jump!