Both the Spanish and Portuguese are partly to blame for the lack of a daytime connection, but to be fair all the railway crossings across the border between Spain and Portugal in the 'interior' are in very sparsely populated areas, so train services are no longer economically viable
You can get as far as Vilar Formoso on the Portuguese side from Lisboa (arriving 13.45) but the there is no longer a daytime train on the Spanish side from Fuentes de Onoro.
This does remind me of one of my best gricing stories though. My dad and I travelled from Lisboa to Madrid on the erstwhile day train in 1986. The daily daytime train in those days was a single two-car Renfe TER (Tren Español Rápido) latterly class 597.
When we got to Lisboa Santa Apolnia in the morning the unit was still in the sidings. WE soon discovered why. The engine would not start! CP then tried to move it from the sidings to the station platform with a Sentinel shunter (CP 1150 class:
http://www.railfaneurope.net/pix/pt/diesel/shunter/1150/nik00782_web.jpg)
Unfortunately due to incompatible couplings the two vehicles then got stuck together for about an hour an a half. Eventually they managed to split them, but the TER engine still would not start, so we departed well late, and hauled by a CP class 2500 electric as far as Entroncamento. More British traction took over in the shape of a CP 1400 which hauled us to the border at Marvao Beira where a Renfe 333 took over. This hauled us all the way to Madrid!
We were over an hour late into Madrid but fortunately it still gave us enough time to have a meal before heading to Cerbere
Those were the days…