I think your last two reasons make most sense;
3) how many stations are active in each mode?
4) propagation conditions may favour shorter transmission bursts rather than a minute transmission
Beaconing is fine but QSOs are what make life more interesting. WSPR15 is supposed to even better on SNR but I really hear anyone using it. I also found where WSPR on 2m yields nothing here, the band is alive with FT8 dx, of which some could be short term aircraft scatter.
73 Ken g4apb