It is interesting to read this post from M1 Finance (available in US) in which they carefully explain the reasons they changed to a $1 minimum investment.
There were previously in a ridiculous position that
Sub $1 trades make up 25% of total trade volume while only accounting for 0.05% of all dollars transacted.
The motivation for change seems very similar to that of Trading 212:
These less-than-$1 purchases can put unnecessary strain on the system for such a low percentage of transaction dollars. Instead of us focusing our development effort on handling micro transactions and system maintenance, we can instead innovate faster and provide more value to you