All a Promo Series does is Limits Your Ranking
Some people might not have been bothered with the promotion series in ranked. Some have had horrible experiences. Lets be honest here, who has actually benefited from a promo series? All it does is slows down the climbing process. You could win 7 games in a row, go 2-3 (total 9-3, 75%) and not make it to the next tier. The main reason I can see that a promo series is stupid is because it is a short group of games that doesn't really relate to your skill. going 200-100 in a season may show you are climbing fast, but going 2-3 doesn't really show much at all