why did 3rd world leaders neglect. agriculture?

There's 2 reasons, one natural, one political.

Most third world countries have poor farmland. Most of these are in tropical zones where soil structure tends to be poor due to the lack of a freeze - thaw cycle which helps keep the soil fairly porous for the roots. Also these soils tend to be very weathered and depleted of nutrients. The best farmland tends to be in more temperate zones. Agriculture is rather hard in areas where the resources are limited, by no means does this apply to all tropical nations, but in general the soil tends to be poorer in these areas.

Also there have been and still are cases where repressive governments will attempt to reduce the food supply in order to create dependence to keep them in power. Get the supply down and create a system where you control a good portion of the distribution, and the people are basically forced into following. Get control of the food supply and they have all the power. Once again not all third world nations are this way but sadly some are, and people die from this manipulation.

They don't understand it and you are in less control of it than other industries.