There are many reasons for people not to spend any time on PGA and its style of program algebra. Of course reasons are not required for ignoring this work. For me it is important to understand these arguments and a listing might be useful for others as well. In practical terms it is a big advantage that no other researchers (i.e. groups outside the UvA) are working on PGA as it makes life more easy-going, and provides a better opportunity to set out a long term project in a number of steps. There is a disadvantage as well, of course. Peer groups in the same area can provide a more productive context altogether and can (but need not!) prevent one from digging into futile depths.
As long as no problem has been solved via PGA that has been left unanswered using preexisting theories the whole strory may be considered redundant. Currently no such solution can be put forward whence the conclusion is justified from the perspective of industrial development.
Inasmuch as PGA-style program algebra aims at improving theory where it is felt to be weak one may evidently object to its methodological 'axioms' or principles itself. In particular the conception of a program as an (unfolded) sequence of instructions may be viewed as low level, PGA may be viewed as 'too imperative', the distance from problematic modern programming features may be considered excessive and so on.
It must be admitted that it has turned out to be harder than expected to develop a body of theory on PGA. In particular it has been much harder to make progress on PGA than than on ACP. But this seems to change. The main reason for this difficulty is that projection semantics is often not very elegant and far less amenable to attractive mathematics than the corresponding model theory of process algebras with elimination results and bisimulation semantics.
By being critical concering the deliverables of existing work the PGA setup does not by itself create a need felt by its audience. Conceptual semantic issues are thoroughly impopular in computer science! To see this one may inspect for instance the remarkable lack of semantic work in the following completely central areas: grid computing, computer virusses, pipelined processor architectures, multi-thread programming. Not even the massive activity in hypertext programming (website design) has led to significant and convincing semantic work that underlies ordinary teaching. Books on webdesign fail to define what constitutes a website and so on. (Now this state of affairs is no reason at all for me to stop working on the subject: things may change and anticyclic work is never stopped by such worries.)