Why don't they try to alternate the types of event girls? 3 out of 4 (or 5) last events were all yellow, as a result I have no use for red and blue manyus but am very far behind on leveling the event girls.

They generally do alternate but I have seen runs of 2-3 of the same type. Are you counting re-issued girls because I don't remember 3 pierce girls recently. Ok, I went back and checked. Last Girls were:

Pierce, Magic, Pierce, Hit, Pierce, Hit, Hit

So I can see your lack of Slice girls... I just didn't notice because of my Gatcha pulls during this time:

Hit, Slice, Magic, Slice, Slice, Pierce, Magic, Hit, Magic, Magic, Pierce, Slice, Magic

huh... seems I've pulled 13 5* from gatcha in the last almost 14 weeks.... Did not even notice till now.

Event girls have a variety over the longer haul.
If you're hard up for slice event girls, the next perma is a slice girl and Hyperwaifu AKA Hypericum is also going to get her perma soon.
Those should help you balance your current type counts out.