When we created Please we were currently using Buck from Facebook. As has been covered elsewhere at the time it had some limitations which meant that it was getting more and more expensive to workaround (no multiple outs from rules so doing things like sourcemaps was hard etc). We had previously migrated from a set of disparate build tools (Gradle, Gulp etc) to Buck after evaluating it against Pants (we actually trialled our repo with both).
We all had experience with Blaze (Bazel did not exist in 2014/2015) and wanted to get closer to the experience we had in Google, hence the final decision to build our own. At the time we rolled it out we had full Buck compatibility (and actually most of our engineers did not notice initially). Once we were happy it worked, we migrated in full to Please.
Overall it allowed us to provide much better tooling for our developers (queries on the build graph for smaller CI/CD footprints, coverage in all languages, and for rules that can output multiple languages (like protocol buffers) we only build variants in the requested languages rather than all languages).
If Bazel had been open-sourced and easy to extend at the time we would certainly have looked to adopt and improve it (as we did with Buck at the time). But by the time they open-sourced it the two system had diverged in their approach and use cases.
I (obviously somewhat biased) think having multiple principled build systems which prioritise the needs of different communities is good for the ecosystem overall (and allows the sharing of good ideas such as the Remote Execution Interface).