OAVs in general have declined over the years since OAVs get less press time compared to TVs or movies. Erratic release times, buying something that no one has seen beyond promotional material,... etc. Chances of a show airing on TV are greater. Chances of a net OAV are greater these days now, but still lower than a TV or movie release. M0 took almost a year to come out from start to finish. To keep interest going, people have to be reminded of it and a OAV can't do if it has an erratic release schedule.