It is a tremendous administrative burden for the IRS to individually tax (and investigate) ~300M individuals. Such taxes also lead to endless political fighting and "back-room dealing" -- and to fiscal cliffs! One could also argue that a federal income tax on individuals is unconstitutional.
It may be possible to fight these economic costs of taxation in support of a federal government by dispersing the collection from the federal level to the state level. For example, the federal gov sets a weighted tax on each state gov (the formula for weight to be determined later by economists/vote). It would then be up to each state to determine how to raise the revenue (such as via income, vice, sales taxes, etc). It would also be legislatively easier for citizens to change their tax system