One big factor is for performance reasons in shell scripts. At the time, the switch decreased boot times for Debian by 7.5%. Bourne shell features add a lot of overhead and that's not always an acceptable tradeoff.
Also, if you're using bash features in a script, you can always just add #!/bin/bash to the top of your file instead of #!/bin/sh to force a bash compatible shell.
https://lwn.net/Articles/343924/
One big factor is for performance reasons in shell scripts. At the time, the switch decreased boot times for Debian by 7.5%. Bourne shell features add a lot of overhead and that's not always an acceptable tradeoff.
Also, if you're using bash features in a script, you can always just add #!/bin/bash to the top of your file instead of #!/bin/sh to force a bash compatible shell.