Gujarat had natural barriers of desert, rann, sea from Sindh which were crossed slowly for trade etc but difficult to surmount in war, Gujarat was semi arid thus not favourite. Entire गङ्गा belt was so fertile and livable that invaders brought afghans,turks, persians to settle. https://twitter.com/porbotialora/status/1301198456510279680

Invading u3 settlements started heavily from West गङ्गा belt itself, it was heaviest there for proximity to capitals Agra & Delhi, settling was even till Varanasi, thinned out gradually eastwards and picked up suddenly in South Bengal and central Bengal (now in beggardesh).

U3 Invaders faced stiff challenges everywhere in गङ्गा belt especially at where सरयू जी merges into गङ्गा जी and there was a huge war of attrtion which DCs won. Such battles happened at रोहतास region where plains meet hills, at Gorakhpur Champaran border where Nepal is close by.

BTW UP, Bihar, united Bengal never had this high percentage of u3 till 150 to 200 years ago during which period Hindu TFR further declined marginally to heavily and u3 TFR had big spurts for decades culminating in partition ultimately. Even during M rule u3 % population was low.

The London School of Economics and Political Science.
PhD thesis:

The Population Trajectories of Bangladesh
and West Bengal During the Twentieth
Century: A Comparative Study.
By Nahid Kamal

Is a comprehensive factual analysis of extensive data.

U3 population in East Bengal rose rapidly in late Mughal period to British era because:
1- EB was less brahmanical, more conversions happened.
2- M rulers gave free lands or tax free in lieu of conversions to encourage agriculture initially less populated EB.

3- Overall Bengal muslims were less educated as well as more into agriculture thus higher fertility rates than Hindus.
4- WB then was unhealthy due to more urbanisation, industrialisation, EB was healthier with lesser pollution & better agriculture. Overall M TFR thus higher.

4, continued- West Bengal had Kolkata, Howrah, Hugli with huge urbanisation & western education leading to low H TFR, M overall avoided western education and were more into agriculture thus increased M TFR as agricultural families need more persons to work in farms as well.

5- East Bengal was hydrologically active thus more fertile and healthier, thus EB saw population explosion of M. WB rivers stagnated due to silt, dams, industry.. cholera, malaria, typhoid were rife; pollution, urban density spread diseases so H death rates were much higher.

5, contd- Death rate Kolkata was 2 to 4 times higher than rest of WB while WB death rate itself was higher than EB which caused Hindu population to fall, barely recovered by TFR.

So for 100 years natural/manmade fluxes caused M population to rise above H leading to partition.