Because if the SP has gone up like it has, shareholders would be...

Because if the SP has gone up like it has, shareholders would be rushing in to get in on the deal. If they extend it to $30m instead of $15m, and 30,000 shareholders all want in, for various amounts from $1k to $30k, then they could only give each person $1k. So if they close it early, then they can give the first 10,000 what they want. The last people buying just before 26th April are more likely than people buying straightaway, to be the ones likely to sell for a quick buck straight after they float. Therefore, it stands to reason that the first people in are more likely than the last people to remain as long term shareholders - beneficial for the SP and the company as a whole to achieve their goals. Furthermore, closing it off early allows them to settle the finance and move forwards. Selling shares isn't their core business; restarting a uranium mine is their core business. The whole point of this is to get a job done, so they'd be wanting to get the whip cracking ASAP with financial certainty behind them after the raise is completed.