I won't tell you it is because of niches. But I will tell you that you are
wrong. Here are the phyla that have appeared since the Cambrian. Some of
them are quite major.
Dinoflagellates Triassic
Calcareous nanoplankton Triassic
Diatoms Cretaceous
Bryophytes Late Paleozoic
Psilophytes Middle Paleozoic
Lycopods Silurian
Sphenopsids Devonian
Ferns Devonian
Cycadeoids Middle Paleozoic
Ginkgos Early Mesozoic
Conifers Late Paleozoic
Angiosperms Early Cretaceous
Bryozoa Ordovician
You are going to now say 'Animal phlya' is what you meant. But you didn't
specify this in your note above. Phyla have appeared since the Cambrian.
And now some phyla are found before the Cambrian like Sponges and molluscs.
glenn
Foundation, Fall and Flood
Adam, Apes and Anthropology
http://www.flash.net/~mortongr/dmd.htm
Lots of information on creation/evolution