Boros is difficult to support as mono white and mono red are more viable options. Lotus petal gives both of these decks a boost to both these aggro decks. Petal being one time use means slower decks are less likely to want it (making it a better inclusion than something like mox diamond).

Adding the temur signets helps the upheaval, wildfire and artifact decks. The multicolour cards being removed just gave added redundancy but did not bring anything unique to signal the pairings.