I think there are many reasons why people may not wish to switch GCs.
One is that their GC feels like a home or their are proud of it.
Gridania felt like a home to me. And so does the Order of the Twin Adder.
I was kind of proud of it.

But when I queued in PvP, we were loosing so much.
In Seal Rock we once only had 35 points when the match was over. 35!
Time and time again, I saw our GC lacked so much organization in PvP. I felt ashamed.

Sometimes only 6 people will join, and 4 are healers. And no one will change jobs...
I wanted to switch, but after all, it is kind of my home.

Another reason, I think, is achievements. Each GC has their own achievements for PvP and other things, supply quests, leves, etc.
When switching GCs, you can't complete them.

I want to at least do as many achievements as possible before switching.

I often want to queue with friends and FC members in FL, but we can't be on the same team. Even if we queue at the same time, we often don't enter together.

Also, even if one person switched GCs to speed up queue times, it probably won't effect much.
Depending on the data center, hundreds if not thousands will need to change.

For example, on Carbuncle server, 42.8% of players are Gridania.
Only 20.3% are Limsa Lominsa.

It's quite unbalanced. Rather than encouraging hundreds of people to change GCs for PvP, removing restrictions makes it much easier to fix PvP.