Suggest that another factor is that low frequencies are not as directional as the higher ones.
As long as the 'sub' can disturb some air, it doesn't make as great a difference as to which direction its pointed. A greater portion of their effect is felt, and not so much heard as compared to the mids and tweets.
On the other hand, the tweeters are usually located so to be a direct 'line-of-sight' to the ears, or reflected off a hard surface such as a windshield.
1989 F150 XLT standard-cab shortbed 4x4, 5.0L, AOD, 3.55 gears, manual hubs, 31x10.50 tires, 218K miles. Repainted Oct09. SOLD - Jan2015
2014 F150 XLT standard-cab shortbed 4x4, 5.0L, 6-speed auto, 3.55 gears, auto hubs, locking diff, trailer pkg, 5K miles.