Probably because as has been said, it would be far too powerful. Juggler is a truly amazing trait as it is, on par with the other advanced traits (or better) yet is also available one experience level earlier. It also has the advantage of being in the WK chain, which is great with all the new assemblies. Nothing fixes poor tactics, weapon choice or reloading like Juggler. Even just the instant shotgun availability is awesome for double/sing/combat decisions and the loaded gun in each hand thing.
If you do have two exotics/uniques then just put them in the equipped and prepared slots. It's not as though you can't immediately switch out to two different "normal" weapons you'd rather use. The BFG9k is an exotic that even has its own quickkey(as is the chainsaw?). I'm a fan of twin combat shottys early on or assault shotty/missile launcher for later myself. Switch in bigger single target guns as needed.
I actually think Juggler could afford to be toned down a little. Maybe make it 0.3 seconds for weapon swaps, or a two level trait with first level giving 0.4 secs and level 2 giving 0.2 secs weapon slots. Fast, but not free. The way it is, it's probably a bit too good. It's probably the only part of DoomRL that allows you to accidentally press the wrong key with no disadvantages, and can keep mashing those keys until your heart's content. It's also a wonderful tactics checker, where you can slowly decide just what you want to blast the baddies with. Every other non-juggler build, you have to know before hand, or pay the consequences of poor weapon choice and reloading tactics.