Are these technical issues that disappear? I don't think so. I don't think the constitution and the division of powers -- however dry it may be to some people -- is a silly issue or so much academic talk. It was, according to some studies, the root of the divisions between Turner and Trudeau and Chretien and Martin and those divisions end up polarizing what was then a functioning party. Moreover, I would argue (I won't here!) that they are important and likely should not be dismissed. My point, of course, is this. Those people who see no differences between the NDP and the Liberals don't know very much about either.
Second, the NDP are in competition with the Liberals on the provincial level in a number of places. The ties between the federal and provincial Liberal Parties are not hard and fast, unlike the federal and provincial NDP parties. Officially, in fact, federal and provincial Liberal Parties are usually (always?) separate parties. Yet, there is an appreciable overlap. (This is the reason right wingers are trying to change the name of the BC Liberal Party, precisely because there is an overlap and they are trying to break it). The NDP and the Liberals are in competition provincially in NS (the NDP seems to have won for now reducing the Liberals to a rump but not necessarily a disappearing one), ON, and BC. The NDP has no provincial precise in QC, PEI, NB and Newfoundland so there is no real competition there. And the Liberals are dead in the water in MN and SK so no trouble there. And, of course, neither does very well in AB. Even in those areas where the NDP does not have a strong traditional vote (NB, Newfoundland, for instance, and AB), my bet is that they will be looking to increase their vote, bringing them into direct competition with provincial Liberals.
Why mention this? Well, this is reason two why the NDP might not want a merger: that merger would be difficult. The Liberals have little interest in signing onto an NDP in NS and ON, PEI and likely NF and NB, among other places. They have a reasonable chance of doing well politically by themselves and even winning (I don't expect the NS Liberals to win the next election but ... perhaps the one after that?).
Finally, why would the NDP want to merge with the Liberals if they had to take over the federal Liberals record? Why would they want, in other words, to take over part (even a small part) of a bad brand? This does not make sense. At the federal level one has to ask: do the benefits outweigh the costs of absorbing some bad history? I won't make that calculation right now but that is not my point. My point is that it is not a lock sync that it does.
So, put all this together and a merger does not look all that smart from an NDP perspective. The NDP needs to take on people with whom they have ideological disagreements, who will support provincial parties with which they are in competition, and who have a bad brand. This does not mean that there will not be a merger. It means that I'd recommend a different strategy to growth for the NDP.