This is pretty late but hopefully still useful to you.
According to gsvelto, one of the developers, it is because the detection of low memory doesn't work:
Yes it does, we've disabled tab unloading on Linux because low-memory detection just didn't work. We will re-enabled it sometimes down the line but only to avoid swap, not crashes. The OOM killer does already a good job with this change and we feel no need to do anything further (besides optimizing memory usage, but that's system-agnostic).