Well his vision was certainly influential. He was a lightning rod of the Arab world for pretty much the duration of his life.

As for why that vision failed, well, for one thing it never actually had any practical institutional basis to succeed. Unions of independent nations, even ones that speak the same language and have similar religious and cultural outlooks are not easily held together without very slow and careful building of national institutions and government systems to bind them together. Nasser didn't pursue that and as a result the UAR was institutionally just Egypt's Nasserist autocracy but now ruling over Syria as well. Needless to say the Syrians bristled at the result and split off after three years.

Nasser's revolutionary outlook also created a sort of a Arab Cold War in miniature as the Arab monarchies saw the free officers movements as dangerous revolutionaries. So you have Nasser investing money, men and materiel into revolutionary South Yemen and getting bogged down into a proxy civil war with Saudi Arabia who likewise invested money, men and materiel into the monarchist North Yemen.

While that was going on Nasser's greatest failure was the '67 war with Israel. While half of Egypt's troops were committed in Yemen the Six Days War was devastating to Nasser's credibility, particularly as Egyptian pan-Arab media was basically announcing incredible Arab victories until the war ended and it suddenly became clear that they'd been completely defeated.

Nasser actually resigned briefly but there was a popular outpouring and he sort of reinstated himself and stayed in office until his death but the dream was basically over.