Developing these islands/reefs costs money, a lot of them. The three currently under construction were picked thoughtfully, IMO. There is no need to invest to build the other four to the same scale; no ROI there.
I think its less to do with money and resources and more to do with long term strategy.
China is using reclamation as a means to send messages and apply pressure as much as it was about the strategic value of having those new islands.
The strategic advantages of having such island bases right at the heart of the region obviously would have factored massively, but I feel a big part, if not the main reason China green lighted the existing projects was to serve notice to some of the other claimants who have been upping the ante lately (notice and contrast how Vietnam and the Philippines were making all sorts of noises and moves before the reclamation, but have calmed right down after the scale of China's operations became evident).
The scale and scope of China's reclamation efforts was a timely abject lesson to those countries of China's power and reach. It was a bit of non-violent shock and awe designed to remind them of just how much more powerful China is without having to resort to the crude measures of bringing out the big stick and waving it about like some countries only know to do.
China stopped because it feels it has achieved all its objectives, but also to leave some room for manoeuvring and bargaining in the future.
If China went and reclaimed all the features it controlled, that's that card fully played and cannot be used again.
By only developing some of them while leaving others largely as they were, China is leaving itself the option of either developing, or using the threat of developing some or all of the remaining features as leverage against future misadventures by other rival claimants.
The message is simple - play nice if you don't want enormous Chinese islands appearing on your doorstep.
I think China will want to leave those islands it has already built with no or minimal military presence for the same reason - leverage. China will be effective saying, "you know those giant new islands we just built? Play nice or they turn from civilian outposts to massive military bases effectively overnight".
I think the main question now is whether the other claimants are smart enough to see the implicit threat, or if they are continue to push and need China to follow through and transform some of those new islands into massive military bases before the message sinks in.