Easy;
Old Hatreds die hard.

Most of the Alliance, which didn't take part in the Hyjal war, still see the orcs as the savages that destroyed their homes, killed their kinsmen, and ravaged their women.

The orcs see humans as the people that held them in internment camps for nearly 20 years, something they'd rather not repeat. Though a special bit of blame is on them since they won't admit that they were wrong in trying to slaughter the Draenei or taking Kalimdor away from the Night Elves, because they high on some manifest destiny which they believe gives them the right to take which was never theirs.

Another part is politics.

When the Forsaken joined up with the Horde, it only made things worse, knowing that the Forsaken are not too far off from the Scourge that brought Lorderon to its knees. The only difference between a slave of the Lich King and a slave of Sylvannas is that one has not free will, while the other only believes they have free will. These people were once living members of the alliance, by the way.

The Blood Elves, being the traitorous little bints they are, blame the Alliance when they left in the first place. When Menethil was murdered, despite everything that Menethil and the leaders of the Alliance did for them.

The Draenei, refugees and not really wanting to become slaves to Sargeras, still dispise the Orcs for nearly bringing them to extinction, when the orcs were too stupid to know the difference between their ancestors and world destroying demons.

Wounds run deep and take time to heal.

The Alliance has a better reason to hate the Forsaken than just their looks. A human army, with help from Sylvannas, actually did manage to retake Lorderon... and then Sylvannas murdered them to the last man. Granted the general was a bit of a prick, but he was a decorated officer and Lorderon was the largest human city since WC2. That's not the kind of thing you're just going to let slide.

As for that truce it was between Thrall and Jaina, not the Horde and the Alliance. Theramore is pretty much its own little outpost removed from the Alliance. Most of the quests there don't even give Alliance reputation.

Just wanted to clarify a point made by tbh: "The orcs see humans as the people that held them in internment camps for nearly 20 years, something they'd rather not repeat. Though a special bit of blame is on them since they won't admit that they were wrong in trying to slaughter the Draenei or taking Kalimdor away from the Night Elves, because they high on some manifest destiny which they believe gives them the right to take which was never theirs."

I dont believe this is an accurate representation of the Horde. Thrall makes a very clear distinction between his burgeoning empire and the Horde of old. He describes his new Horde as different from the brutal murderers of the past, and as such I believe he would view the slaughter of the Draenei as a contemptable act.

Similarly, I believe Thrall wasn't so much looking to take Night Elf territory, more like they were plunged into hostilities by forces beyond his control. Thrall didn't know that the Night Elves even existed when he ordered the Warsong clan to start chopping down trees building fortresses, and the whole thing was made even worse by Hellscream's second corruption and the following slaughter of Cenarius. Effectively the Orc-Night Elf feud occurred without Thrall being involved at all. I believe that if he could, he would be perfectly willing to find a way to coexist peacefully. After all, he made peace with the Tauren and native Ogres quite happily. He also happily made peace with the Night Elves and Jaina's forces of humans when called upon to do so. It was the subsequent actions of Daelin Proudmoore and other political events that led to the continuing hostilities.

So, whilst the Horde are not exactly wusses, and admittedly have been ratehr expansionist towards local Centaur and Quillboar, I dont think its accurate to lay this kind of blame at their feet.