That's because Bangladesh has way too many polytheists, mushrik and pagans.
It is not an Islamic state ruled strictly by Islamic Shariah.
Generally, the "more Islamic" countries are "more developed", at least in the limited region stretching from Morocco to Indonesia.
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Case in point.
Maldives - 100% Muslim population, higher GDP per capita than Sri Lanka, higher literacy rate than Sri Lanka, etc.
UNICEF - Maldives - Statistics
UNICEF - Sri Lanka - Statistics
Maldives and Islam - 1 | TurnToIslam
This is exactly why Brunei and Malaysia are much more developed than Indonesia, even though all three countries are generally located in the same region, and consist of the same majority religion, and similar cultures.
For example,
Brunei's GDP may rise by 2% | The Brunei Times
The Brunei Economic Development Board
Malaysia is neither a decidedly Islamic state, nor a secular state like Indonesia. Therefore, Malaysia is an upper-middle income country, neither very rich (like Brunei) nor very poor (like Hindustan or Indonesia, in relative terms)
Malaysia's per capita income rises - Latest - New Straits Times
In Malaysia, due to 'too much diversity', Syariah (Shariah) is
not
given precedence over (kafir) "civil" laws.
Syariah laws in Malaysia BorneoPost Online | Borneo , Malaysia, Sarawak Daily News | Largest English Daily In Borneo
Indonesia is the poorest of the three Muslim majority countries in ASEAN, for obvious reasons (kafir laws/secularism).
Indonesia Remains Secular Despite Islamic Revival | Online NewsHour | October 22, 2009 | PBS
Indonesia's per capita income grows 17.7 pct in 2011 - Xinhua | English.news.cn
Ergo:
The more 'kafir/secular/polytheist/pagan/etc' laws, customs, cultural elements, practices or beliefs are embedded in a Muslim majority society's laws, traditions and culture, the poorer and backward it is.
And vice versa, the more Islamic a Muslim majority state is, the wealthier and more developed it is.
Reference:
The evidence presented above from a variety of highly respected, verifiable and authentic sources.