You would think that event priority would be a feature that would make sense for them to include (or any number of other differences in how unlocking girls could have been handled). But the way they actually set it up, it's just a matter of beating the (sometimes very harsh) odds to not only get a drop but to also have it happen to choose for your drop to be the event girl as well. Probably a scheme to encourage people to shell out tons of money to just get the drops through epic pachinko spins instead (which is way overpriced to get the guaranteed drop (for a single girl per 10 spins) and may or may not feature event priority (I think it's supposed to, but there have been reports of it not working and just giving out whatever random regularly available girl instead of an event girl)).

* My random thoughts on a potential alternative unlock possibility:
1) include somewhere an inventory where you can see collected boss item drops (the ones needed while going through the story), which will continue to be possible to receive after finishing the chapter
2) make use of the suggested inventory space to also be able to collect event tokens when an event is running (similar frequency as the boss item drops or maybe slightly higher, each different boss participating in the event drops the same event token)
3) add some sort of shop or trade button on each chapter (next to the boss's fight button, I guess) where the chapter's 3 girls and maybe some other extras (random item box (or a ticket for a free great pachinko single spin)? a bit of xp or mojo? a ticket (however expensive) for a free epic pachinko single spin?) can be bought with some number of the boss's special item drop (may require a bunch of them (a matter of weeks worth of farming the boss) to get a single girl, but it's something to work towards if luck isn't giving a girl to you as an instant win, and the extras keep the boss items (somewhat) relevant on a permanent basis)
4) make use of that shop/trade option during events to let us buy event girls (in the appropriate chapters' shops) and the same types of extras using the event tokens (with the event girls hopefully priced fairly reasonable since there's only a couple weeks to get both captured event girls)

A system like that is still tied into a bunch of RNG (which could see people having bad streaks, just the same) but it would give people an alternative option with a measurable amount of progress towards getting regular and event unlocks.