Why is 1-bromo-3-methylbut-2-ene named like that and not as 2-(bromomethyl)but-2-ene? I had this confusion related to nomenclature of halo substituted compounds.

Why is it that 1-bromo-3-methylbut-2-ene is named like that and not as 2-(1-bromomethyl)but-2-ene? why do we prefer the carbon chain including the carbon attached to the bromine and not the chain including the carbon of the methyl group? 

Which IUPAC rule was used in making this preference between those two carbon chains of equal length?
Here's 1-bromo-3-methylbut-2-ene's structure:



You have only two possible chains to choose from: a 3-carbon chain and a 4-carbon chain (doesn't matter which methyl group would be part of that chain, the other one will become a substituent on the chain). 

You won't choose the 3-C chain for two reasons: 


It is not the longest chain.
It does not contain the double bond in the molecule. 


Thus, you are left with a 4-C chain, which necessarily includes the bromine atom.