I always wonder, why in GIS industry there are merely two major options available. Although there are many other proprietary software products for GIS, their market share is well below ESRI products (here, here and here although quite back dated). My opinion became stronger after joining GIS SE. Most of the questions, as well as the leading users (with highest reputation), are either for/interested in ESRI line of products or FOSS (of course apart from some questions related to data and theory).
However this not the case in other streams of information technology, databases and programming languages in broad sense. I personally feel one reason could be the lack of interest by the software giants like Microsoft, Google or Oracle. While the first two have WebMaps, these are not WebGIS, and the spatial extension of Oracle is not fully GIS too.
What are the major reasons for such a trend? There is nice article somewhat related to this, " The Five Major Roadblocks to GIS Gaining Corporate Market Share". But this is not the answer I am looking for.
EDIT: I am afraid. Most of the answers seem to prove open source tools are better than ESRI products! But that was not the question. Because it is difficult to compare a matured but expensive tool with a bunch of comparatively newer open source tools with great usability. My question is why no industry standard product from any established software giants? Hope this helps...