Why hasn't the discovery in 1998 of the increasing rate of the expansion of the universe had a bigger effect on the philosophies of science and astronomy?
It did have a huge effect on the philosophies of science and astronomy!
First of all, what do you mean by "bigger effect"? It did have quite a huge impact in cosmology and astrophysics that we had to rewrite entire libraries with the new discovery and its implications. Nowadays, all astronomy books include this unexplained "dark energy" and the mass-energy density pie-chart of the universe had to be revised.
Perhaps you felt it wasn't as big as the paradigm-shift that occurred in the early-1900s when Edwin Hubble proved that Andromeda and other spiral clouds were in fact entire galaxies similar to our own (which made the known universe to substantially increase in size, plus its expansion meaning that the universe had a beginning), or as big as the discovery of the Cosmic Microwave Background in the 1960s (which proved once and for all that the Big Bang really occurred and that the universe had a beginning).
The omega value, or critical density of the universe, is very close to 1 (i.e. flat universe), and since the expansion rate of the universe seems to be accelerating, it will keep expanding forever and faster and thus its geometry MAY be slightly open (but we don't know that for sure and it is up to debate, and if that is the case, the extent of the universe beyond the observable one could be infinite).
I realize that I need to explain more in my answer about another question asking for the volume of the universe, to which many of us replies that the "universe is infinite", as it would only make sense if the geometry is either really flat or open.
The current standard model of cosmology that best describes the universe is called the lambda CDM model and takes into account the cosmological constant (first introduced by Einstein but to have the opposite effect of what it is used for today, the dark energy). There are many references in the literature (perhaps not popular science), but I find the WMAP and Planck websites to be quite good to get a grasp of the current model. That's why, it still had a great effect.