A predictable monthly amount makes sure that Dash’s stakeholders, as well as all proposal owners, can budget ahead of time, rather than getting blindsided by a big DIF ask during a single month. The more predictable that treasury asks are, the better masternode owners/shareowners can do their jobs.
We must prevent a scenario in which the DIF is forced into a breach of contract by the network. This could happen if the Dash network first agrees to pursue an opportunity (for example, via a multi-month funding proposal specific to a single investment), but then de-funds the agreement before all payments are made. This could happen even if the proposal doesn't lose any votes over time, but is simply surpassed by other projects which come to have more support.
Remember that the Dash network is a fluid body — its stakeholders change month-to-month — and the DIF must not risk breaching legally-binding contracts with outside companies due to the network shifting its priorities, which has happened in the past.