Does anyone here know why the He P.1078 (presumably all derivatives included) got cancelled? The only place I could see for sure was unfortunately Wikipedia, which states that "severe criticism" caused the design(s) to be cancelled.
The whole story of the Heinkel P 1078, based entirely on primary source documents, is in my book Secret Projects of the Luftwaffe Volume 1: Jet Fighters 1939-1945.
Here's a potted version...
To say that it was 'cancelled' isn't really correct; it would be more accurate to say that Heinkel's tender for the Chef-TLR's advanced single jet fighter specification was declined.
The process of providing a new aircraft for the Luftwaffe in Nazi Germany usually went as follows:
1 Requirement from the Luftwaffe. The Luftwaffe, usually through the OKL (Luftwaffe high command), would tell the RLM (German air ministry - latterly, for our purposes, referred to as the Chef-TLR) that a new aircraft was needed to fill a particular role and indicate what its desired performance ought to be.
2. The RLM would draft a specification based on this requirement then issue that specification to certain aircraft manufacturing companies, inviting them to submit designs intended to meet the specification.
3. The aircraft companies would go away and work on their tenders. This is the point at which the 'projects' were usually started. Sometimes the specification was very, well, specific and the project from different companies turned out looking rather similar. Other times the specification was extremely vague, resulting in widely differing designs.
4. A comparison meeting would be held where representatives of each tendering company would be brought together with RLM engineers and other specialists to compare the designs against each other. If the designs were similar, the comparison was relatively straightforward. If not, it could be very difficult to establish a good basis for comparison. At this stage one or more tenders could be rejected, with the most promising designs moving forward to the next comparison meeting.
5. Eventually one or more designs would be chosen for development and the successful company would be issued with a development contract. This was usually for a short run of prototypes on the basis that should these prove acceptable a full initial series of production aircraft would follow.
The advanced single-jet fighter competition (which would eventually include the P 1078) was run in parallel to the basic/economy/emergency Volksjaeger single-jet fighter competition. Heinkel had already won the latter with a downgraded version of its P 1073 design (originally submitted for the advanced fighter contest) and was specifically told that the original advanced version of this design would no longer be accepted for the advanced competition.
So Heinkel had to literally go back to the drawing board. Based on its experience with the He 162 (being aware of how tenuous the situation had become with regard to manufacturing and raw materials) and having seen what other companies were offering for the advanced competition, Heinkel created the extremely compact but aerodynamically advanced P 1078. But the other companies all now had a decent head start in terms of paperwork and Heinkel was unable to supply the mathematical calculations necessary to support the P 1078's aerodynamic design. It simply didn't have time to work through the maths, whereas the other companies could at least supply some data in support of their tenders.
As a result of this, the P 1078 drew criticism and was not among the designs that the adjudicators felt were most likely to succeed. As of the end of February 1945, the P 1078 is no longer mentioned in what documents concerning the competition survive. It would appear that the P 1078 was declined at this point because the leading designs (such as Focke-Wulf's 'Huckebein', Messerschmitt's P 1111 and Junkers' EF 128) were simply better worked through and were therefore more likely to succeed.
You mention derivatives - there were no derivatives. Only one P 1078 design was tendered for the competition. For more clarity on this point, read the book.