Randman - let me try to explain what Modulus is saying to you, as I don't think you have got it.
Phylum is a taxonomic rank. (plural=phyla). Taxonomy deals with classification (at least traditionally) and only secondarily with evolution. Modern taxonomists try to classify according to evolutionary relationships, but the taxonomic system (which goes back to Linnaeus) is in many ways imperfect for this. Still, we need to be able to refer to particular groups of organisms using terms that are unambiguous and universally understood, so the system survives.
The distribution in time of the origins of the various organismal groups corresponding to phyla has less to do with the frequency of major new body plans arising and far more with the problems in the imposition of a /ranked/ hierarchy on life, which is fundamentally a nested hierarchy without ranks. Think about the famous birds/reptiles example. Birds represented a significant new body plan arising from reptiles, and in traditional taxonomy are recognised as a class (as are reptiles). This means that reptiles are an artificial group, as the particular group of reptiles from which birds evolved share a more recent common ancestor with birds than they do with the rest of reptiles.
It's the same with most of the phyla. They didn't all arise equally independently from each other, so for example vertebrates and echinoderms (starfish etc) share a common ancestor with each other more recently than either do with arthropods. So "phylum" doesn't really mean very much, and most people interested in evolutionary history these days don't really talk about phyla, they talk about echinoderms, vertebrates, the group that includes enchinoderms and vertebrates, the group that includes arthropods and velvet worms, and so on. In defining phlya, traditional taxonomists looked for the most inclusive groups, i.e. the ones that had other definable groups nested within them (classes, orders families etc), and because generating diversity takes time, these necessarily tend to be older clades. Simply, the requirement of the system to have ranks meant that the most inclusive ranks pretty much had to be old.
I think the question you are trying to ask though is "why did all the major body plans arise in the Cambrian (or whenever) and not continuously throughout time?" - and as has already been pointed out, they didn't, they have popped up all the time but aren't called phyla, because the later they have arisen the more nested they are in the hierarchy. If something equivalent to a new phylum in the way you understand it popped up tomorrow (or a mere million years ago) it wouldn't be a new phylum, it would just be a highly unusual member of an existing phylum. If it so happens that in 500my time it has diversified into a major group with definable characteristics, its relationships to all other groups have become obscure through extinction and other processes, and there are still Linnaeun taxonomists around to think about it, then maybe it will become a new phylum..
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