Ann: Boss set to become a multi-mine uranium producer in 1H 2024, page-34

At least part of the reason for that is that they need X amount of guaranteed cash to do what they want to do.
They can ensure they have enough money if they get that guaranteed from the big players.
They cannot ensure they will get enough buy in from retail shareholders so they can't risk assuming they will.
That's also why the $10m raised from retail isn't being used for the main purchases. It can't be guaranteed to be all sold.

A lot of retail will go for whatever they can get at the best price. They are in it for themselves more than the company they are investing in (sounds funny but that's often the mentality). So when there's an SPP at X per share then the market value inevitably drops below that a majority of retail investors will buy on market at the lower price, instead of supporting the company to raise the extra cash and paying a bit more.
Depends how much the company needs the money and how big the gap is between market and SPP as to how negative that is for the company.

So it's not really avoidable to raise from the big players if you need to guarantee a large amount of money coming in.