Because the fastest attack & release settings of the native Audacity compressor are too slow for speech, e.g. the shortest release-time is one second: the compression takes a full second to wear off, (that’s longer than the interval between words).
There are free compressor plugins which do a better job on speech …
Bertom (simple) vocal compressor works in Audcaity3
TDR Kotelnikov (complex, but has vocal presets) works in Audacity 2 & 3