The maximum number of shares that could be issued under the SPP...

The maximum number of shares that could be issued under the SPP was specified in the prospectus, and in the AGM resolution that was passed on Monday. The whole thing was subject to that resolution getting approved, so the Board can't just decide to issue more now.

You could ask why they didn't make the total amount higher in the first place, but bear in mind that the share price movements after the insto CR were very unusual. Nobody would have expected the SP to go up to over 30c straight after they announced a CR at 20c. Most of the time, the SP trades down to around the CR issue price. If that had happened in this case, then it's unlikely that they would have even raised the $2m they were seeking, never mind received applications exceeding $4m.