R’ Soloveitchik explained that there are 2 dinim in aveilus: there are nihugei aveilus, the do’s and don’ts, and there is the shem aveil on the gavra.
Even though according to the Rambam m’doraysa there is only one day of aveilus, the Rambam brings l’halacha the din (M.K. 15) that an aveil may not send korbanos to the mikdash for all 7 days. Even though m’doraysa the nihugei aveilus apply only on one day, the shem aveil is there for all seven. (Rav Wahrman in his She’eris Yosef vol 2 gives a different teirutz: even if aveilus is derabbanan, the person is not thinking straight so there is a chisaron in da’as ba’alim of the korban. However, see the Ritv”a who holds the whole din applies only to shelamim but not chatas – tzarich biur.)
When there are no kerovim the halacha is that you still have 10 people practice nihugei aveilus for the meis – there is obviously no shem aveil on the gavra, as the meis is not related to any one of the 10 and they don’t feel the same as real aveilim, but still, there is a din of nihugei aveilus.
R’ Akiva Eiger in Y.D. 388 has a safeik in the case of someone who became an aveil on Y”T, in which case the aveilus is pushed off until after the Y”T ends, whether the person wear tefillin on the first day he observes aveilus post-Y"T. The way the Rav understood the safeik (though see R”AKE inside) is whether the issur of tefillin is a din in the shem aveil, which sets in immediately on Y”T, or whether it is a din in nihugei aveilus, in which case it would not apply until mourning is actually practices, the first day after Y"T.
Assuming the ptur of tefillin only applies where there is a shem aveil, we understand why it does not come into play on Tisha b’Av. We practice nihugei aveilus, but there is no shem aveil on aveilus yeshana – it simply does not have the same effect on us. We mourn, but it is a mourning of a historical event, not a mourning of tragedy fresh before us.
We still have a day before the dast, so let us hope that Tisha b'Av this year will become a day of sason v'simcha, and our kinos will be transformed into a true tikun (Likutei Moharan 247 -- can I quote that in the same breath as the Rav's torah? : )