If you want to understand why it was decided to use approval voting for witness voting (and explicitly to not have a downvote option), please read this: https://bitsharestalk.org/index.php/topic,5164.0.html (If it is not already clear, @bytemaster in that BitSharesTalk thread is @dan.)

Downvoting was part of the original design for DPoS. But then an attack scenario was considered that led to changing the design to approval voting.

Also, with approval voting you should ideally have no limit on the number of witnesses you can vote for. (That means going to the other extreme of infinity unique votes rather than 1 single vote that you split up.) Unfortunately, the introduction of the time-share witness in Steem meant that there needed to be a strict upper bound on the number of votes, otherwise an attacker could use sockpuppets to exploit it to completely dominate the time-share witness slot. (Note: some reasonable upper bound was needed anyway for memory reasons, but the stricter limit of 30 is a compromise, introduced in hardfork 2, between the competing needs of approval voting and reducing the abuse of the time-share witness slot through the sockpuppet attack.)