Was there any particular reason why WWE took a 2 year break from big dome shows in 2005/06 for WrestleMania 21/22? It seems odd that they were in the AstroDome, SkyDome and Safeco Field for that string of years from 01-03. Then, for obvious reasons they went to Madison Square Garden in 2004. They took the two year break and then headed to Ford Field in 2007. Since then, they haven't looked back and have all been in stadiums or big domes.

Was business just so low during that period?

Well the thing is that they have to book stadiums WAY in advance, so it's not like they can just pivot and run a huge show like Wrestlemania somewhere else.  Plus I'm pretty sure they were getting a shitload of money to run in LA in 05 anyway.