Partner has opened a 5 Card Minor and shown a 4 Card Minor.  Although I do not fault your rebid of a 5 Card Major as Responder, the odds are you cannot expect much in the way of Spades from partner.  At this point, partner does not know if you have more than a scraggly 6 points.  2NT corrects this so that you both know game is possible while potentially limiting your hand to no more than 2 Hearts and 3 Diamonds with a Club suit you don't want to (or can't) bid.  I feel this is much more descriptive of your hand as you really don't want to go to 4 Spades with partner holding as few as two trumps.  3NT is the contract you want to aim at and aggressive players who won't want to risk (a new) partner passing will be bidding it now .... which makes 2NT your minimum bid at this point.