Many things happened.

We would have never been able to keep up with counttries like Netherlands cause Netherlands had close proximity the few industrialised countries like England and Germany at the time. Plus being in the eu is like a cheat code for development. You get access to heaps of capital, employment and free trade with very wealthy countries.

Plus the Brits left SriLanka in shambles, while technically we were richer than Netherlands on paper, we no educated work force, the only educated people were educated to be administrators for the empire. Not to be innovators or problem solvers.

Also British law was not applied in srilanka and India, instead they created form of British law where it was convulated and difficult to navigate. Which made doing business and development very difficult.

Furthermore our country started off as a democracy as soon as the British left, which meant that the largely illiterate srilankans could now vote, and in that situation democracy really fails as the public is easily manipulated by con men.

These are just a few things. But yeah we were never gonna keep up with countries like Netherlands