Baically, Mono U is focused more toward lategame advantage, and early game one for one trades, and after the first turn, sol ring > petal, so the few times the lotus would be better doesn't make up for the fact that in the majority of cases the ring is strictly better.
espically since only 4-5 of the 12-13 counters have double blue (4 drain, maybe 1 counterspell). Mana leak only needs 1 blue and by the time you get 5 mana to hardcast Force, lotus petal vs sol ring wouldn't matter.
Plus land, solring, turn 2 land, phid/B2B, turn 3 have 3 land and sol ring open seems much better than turn 1 petal, land, turn 2 land, phid/B2B, turn 3 have land open.
Bill