what are the two main reasons why Hadleys single cell" air curculation theory does 'not work?

Answer 1

The earth rotates too darn fast...if the earth rotated slow enough then models predict that a single large Hadley cell could set up-- as opposed to the smaller cell that we see in the tropics & subtropical regions today.

I don't know about reason #2, I'll guess it has to uneven distribution of land between hemispheres or seasonal variations.
Check w/ your text to verify, please...

Answer 2

In a single-cell model, the rising/falling branches of the Hadley cell are at the equator/poles, due to more/less solar insolation. Upper level winds move from the equator to both poles, but to conserve AM, as r decreases, v must increase. This would create extremely large westerly winds that are dynamically impossible. To conserve angular momentum, a 3-cell approach is now used consisting of a Hadley cell, a Ferrel cell, and a Polar cell.