If I was guessing they were not able make enough stock and knew it. That's why they are not releasing the numbers sold for the preorders. Overall tho if your ordering a new colour they have always been in really high demand just like the white,gold,rose were back when they were released.
I don't think it's quite as simple as that though... Lot's of people always moan about Apple (and other companies releasing popular products) restricting numbers, or not releasing numbers.
In truth, when it comes to manufacturing you have to have a sustainable model - sure they'll be a ramp up at the start of production, a period of higher production, then a ramp down to a lower sustainable normal production.
Whilst you're stockpiling though before going on sale and during ramp up, you need to be able to store raw materials, employ staff, store completed products, and then ship them. All of those things cost money - and the more you stock pile the more space, materials and staff you need. The more courier capacity you need. The more storage your retail partners and carriers need. If you make enough to meet initial demand for day one or week one, then capacity in all those things needs to be massive - to then have 80% not be needed a week later.
From a business sense, it's not really viable - and even if Apple could do their bit, their suppliers and couriers probably couldn't do theirs. A more sustainable model across a longer period of time works better... Not just for Apple and their partners, but even for people hired. Sure the "meeting demand for day one/week one" model might create more jobs - but it'd be for a much shorter period of time with higher layoffs -- a more sustainable model creates less overall jobs, but they're more permanent jobs and more longer-term contracts.
You then need to look at something like the iPhone 7/7 Plus. There's two version (7/7Plus) in 5 colours in 3 different storage sizes all in 3 different carrier bands/techs. That's 90 SKUs. That's an enormous number of different products!
Everything Apple did to stockpile these different products before release would be based on educated guesses. Making 7 Pluses is likely to take longer and definitely uses more materials, so the numbers manufactured in the same time period would have to be lower (the 7 and 7 Plus are almost definitely separate production lines so you can't just switch capacity from one to the other).
Of course, once pre-orders opened, Apple gets an idea of which of the SKUs are in higher demand, and they can adjust production accordingly. Maybe not between the 7 and 7 Plus, but between the 45 variants of each in order to meet pre-orders as fast as possible, whilst making the manufacture and shipping process as sustainable as possible....