There was a big push a few months ago to add many of the missing cards to Arena for pioneer. This coincided with a period of almost no pioneer events though and then no more cards got added. Now there is finally another Arena pioneer event but there are still lots of cards missing.

Skyline cascade would be a really huge one. It's proven itself a modern staple in the amulet deck and quite good in the scapeshift combo deck in pioneer

The fun part of magic is that there are countless unexplored cards and strategies. The cards that don't currently see play are so important to keeping a format dynamic. It is very engaging to find the perfect card for the moment and that requires having more than just the cards that have proven themselves.