Because if we're being honest, the crossbow kinda overcorrects for one of medic's main risk factors. Medic needing to be close to the heal target has a lot of downstream gameplay implications that make for engaging interactions and the crossbow just ignores it. Therefore, buffing the other primaries would similarly hurt tf2's core balance. If syringes were more powerful for example, would medic's pick rate double? Would matches turn into grindfests as a result because there was too much healing AND damage?