Horner, the oldest winner of any grand tour after victory in the three-week Spanish race last September aged 41, left the American RadioShack squad at the end of 2013.
Barring a last-minute signing, the Oregon-based rider will not be at the start of the race in the Andalusian town of Jerez de la Frontera on August 23.
"We invited him, and he is totally welcome. We are very proud of our winners and Horner is the current defending champion," Vuelta director Javier Guillen told journalists.
"Maybe he didn't want to come because he has no team, but I really don't know the reason."
Alberto Contador of Spain, double Tour de France winner and a former team mate of the American, looks the favourite for the 3,181.5-km race this year
"See you in Jerez next August 23rd," Contador, who won the Vuelta in 2008 and 2012, told reporters in a televised message from his Canary Islands base.
Eight summit finishes, starting with a 5km ascent to the mountain town La Zubia in eastern Andalusia on stage six, will favour the climbers.
Stage 11's 10km ascent to San Miguel de Aralar in the province of Navarre on narrow, exposed cement trackways could separate the favourites from the also-rans.
The Vuelta avoids Catalonia's Pyrenean climbs but features an ascent to the Lagos de Covadonga mountain range In northern Spain, home to some of the last wolves in Europe, at the end of the second week.
The climbing concludes with Stage 20's ascent of the Ancares in Galicia, 24 hours before the finishing 10km time trial in Santiago de Compostela.
It is the first finish outside Madrid since 1993.
"That last little time trial won't make much of a difference," Contador said."With so much climbing, the race will be decided well before."
- Sports & Recreation
- Alberto Contador
- Jerez de la Frontera