a) The existing plant is located on top of the new Grace...

a) The existing plant is located on top of the new Grace resource. It was old when the mine shut down last time and has not worked for almost 20 years. It is more than likely no more than scrap metal now and could cost more to dismantle than it is worth in scrap.
b) The existing plant is not designed to process the type of ore that Grace represents
c) They dont know where to put a new plant - one that is able to use modern technology which is significantly better than 40-odd years ago when the existing plant was designed. They wont know until they have drilled enough barren ground to place a plant, waste dumps and tails dams without sterilising potential new open pits.
d) They dont know what size or type of plant they should build - This will require a number of rounds of studies and metallurgical testing. It is possible that they could try mining some high-grade material and processing it 100km or so away but that is inefficient. They could also build a tiny modular plant and see if they can make a bit of money but it is also inefficient and would still require a significant amount of permitting which in turn would require a fairly detailed level of design.
e) They dont have a JORC-2012 compliant Mineral Resource. Without that they cannot prepare an Ore Reserve, it is illegal. All they have is an underground MR that is (probably) either JORC-97 or 2004 compliant and related to the refractory material below and around the old underground mine. JORC-2012 included enforcement of the amount of detail and work required to confirm MR/OR estimates, like about 10 times as much as what people were able to get away with before then. Without an Ore Reserve they cannot go to any banks for money because they would be laughed out of the room.
f) They dont have the money - a new plant will cost $50-150M, the studies and additional drilling required will cost maybe $20-50M and take 2-3 years if done properly. They will cost more in the long term if short cuts are taken, only it will look cheaper. Other capital items like cut-backs, new camp, etc... will cost a bit too. Probably in the order of $120-350M in total. Alex is a geo with a grad dip in finance - It is very unlikely that he understands these things but he probably thinks he does or is only just recently discovering how much he doesnt.
g) They dont have permission - There is an existing mining lease which makes things easier but only marginally. Most of the permissions are environmental and native title these days. They cannot begin to seek permission until points a-e above are mostly complete.

There are a few other reasons but they'll do for starters. I could be corrected on some of the above but it is how it feels to me.

Having said that - the project looks good. The announcement today is way overdue and not terrible but not that good, which more than likely explains why it took so long. Normal procedure there - put out anything good immediately, delay less positive news as long as possible in the hope that something positive might come along in the meantime. Happens hundreds of times a year and the ASX does nothing about it. Overall it looks like things might be pushed back half a year or so as a result but it still looks like a decent project could come out of Youanmi.

All IMO and DYOR of course. Not advice.