You had mentioned two things, the lack of marketing, the low number of copies produced, the bad release dates and how Shadow Dragon and New Mystery were supposed to be the last games in the series before Awakening.
The first point, I could perhaps understand, but again, only for North America and Europe. FE wasn't an established series, so Nintendo didn't see much of a reason to support it as much as their other franchises. Still doesn't explain Japan though.
And in regards to Shadow Dragon and New Mystery possibly being the last games in the series before Awakening, they both SD and New Mystery eventually sold over 250 k copies, which was infact the minimum number of copies that Awakening was required to sell in order for the series to continue. Therefore, I strongly believe that Tellius doing as poorly as it did is what even made Nintendo consider cancelling the series in the first place.