I think the real question should be, how have those other countries managed to do even more badly than Sri Lanka?
Assuming the figures from here are correct (click through the individual countries in the table near the bottom) then the 1970 GDP/capita figures were:
Sri Lanka - $197
India - $111
Pakistan - $222
Bangladesh - $105
Over 42 years a tenfold increase in GDP/head is a miserable performance, especially for a developing country. The UK managed double that in the same period despite being a mature economy struggling with deindustrialisation and whatnot.
For comparison Malaysia has gone from $343 to $10,422 in 2012 (30x increase). Vietnam went from ~$63 in 1970 to $1716 in 2012 (27x). Being outperformed by a country that suffered a devastating war and decades of hard-core doctrinaire communism is a pretty good indicator that things are not going well. For real comedy, China grew GDP per head approx 54 times in the same time period. It started at $1 more than Indias and ended up at $6K versus $1.5K.
My guess - while Sri Lanka has performed dismally over the last half-century or so, the others have done horrendously badly to even more conflict, corruption, and cack-handed political meddling in pretty much everything than has occurred in Sri Lanka.
Ps. That site also has lots of nice country and region graphs showing “The World” accelerating off into the distance (if you look at e.g. South Asia or Africa) or The World being left far far behind (if you look at Europe, Americas, or specific countries like Singapore, Korea etc). Also some nice precalculated “what if X were as rich as Y” comparisons.
As per 2011 census, TFS for India was 2.5x ( was >3 in 2001) . It should be lesser than 2.5 atm. Read somewhere that Govt. has a target for reaching replacement rate (2.1) by 2017 but ofcourse population will start to stabilize many years after reaching 2.1 somewhere at 1.5 billion. See state-wise TFS here (Many relatively prosperous states are below 2) -