Why do these other designs not fit?

Try this: Modify the file named "top_synth.qsf" from your BSP and add the following line:

set_global_assignment -name AUTO_PARALLEL_SYNTHESIS OFF

This will disable parallel synthesis for Arria 10. I had an issue like yours that a design with very low area utilization failed to route on Arria 10, while it worked fine on Stratix V. After lots of email exchanges with Altera's support, they found a bug in their synthesizer for Arria 10 which is hopefully going to be resolved in v17.0. They recommended the above workaround for now.

If the fix doesn't work for you, try to compile your design against Altera's reference BSP for Stratix V. If it still doesn't route, there is some issue you have to fix in the design itself.