Why was the banding ability 'removed' from MTG? What was so complicated about it?
Take it from the horse's mouth:

The short explanation of banding is that if your creatures attack or block in a band, the controller of any creatures that deal combat damage to them doesn’t get to assign that damage -- you do. Pretty cool. Everything else about banding, though, elicits a “huh?” It works differently when attacking and defending. (In an attacking band of X creatures, at least [X-1] of them must have banding. In a defending band, only one creature must have banding.) A lone blocker could block multiple creatures if they were banded together. If an attacker with evasion (flying, swampwalk, whatever) is banded to a creature without evasion, the whole band can be blocked. Then it gets really complex: What if the only defensive creature with banding is destroyed before damage is dealt? What if my band of three attacking creatures, one of which has trample, is blocked by your band of three defending creatures, one of which has first strike?
The only answer I can give you is, “I dunno.”

Banding was an ability that was flavorful and simple enough in the simple cases. But when you left the simple cases and got into more unusual situations, the rules suddenly became very complex, unintuitive, and opaque. You wind up with a mechanic that cannot be understood or accurately applied simply by reading the text on the card and applying your general rules knowledge. Instead, you have to know a whole bunch of banding-specific rulings, and hope you have a copy of the comp rules handy if it ever comes up.
And all of this complexity is maintained for a mechanic that rarely actually mattered. I could be wrong, but I don't believe there is any card with banding that has ever been a tournament staple. Under these conditions, the best choice is the one that Wizards made, which is to let banding lay fallow and pretend the whole thing never happened.