Because it's 'easier' to play mono-color cards reliably? Kinda makes sense to me.
FWIW, I am not convinced that mono cards are easier to play. But multicolor cards do force specific color combinations, which may justify the added power. I can't do it right now, but I read a very interesting article on the subject a while back. The were talking about GG vs GW 2 drops, and why it was actually more difficult for a competitive deck to reliably cast GG cards (obviously
or
are easier to play than
, but that's a false equivalence, you must compare to
or
). Basically, they were going over the math, but I can summarize it here:
You need 20 G sources to run GG on t2 vs. 11 G and 11 W to run GW on t2. So on the surface it looks like the GG is slightly easier (it requires a 20 card commitment and the other requires 22). It isn't however, because say you run 4 GW dual lands, you know only need a total of 18 lands to meet the GW card's requirement (4+7+7=18). This would hold true even if you don't follow the hypergeometric distribution theories on how to build decks - and use some other method to build you mana bases, you'd still have some starting point and the dual lands would make GW cards easier than GG, or WW.
On top of that, the majority of the time you are going to run 2 or more colors, so the GG requirement is an even bigger deal, from the perspective of deck building. GG forces you into mono, or maybe dual color decks. GW is believable in 2, 3, and possibly even 4, or 5 color decks.
It gets even more obvious with a card that costs GGG, vs a card that costs GWU - just as an example.
I actually think WotC has it exactly backwards - but for the record the above is the entire reason I believe this to be the case, so if you don't buy into the logic, I unfortunately don't have much to add.