Why some phones don't is a mix of history and whatever motivations manufacturers have for their choices.
The basic "move to SD" was introduced around android 2.1 or 2.2, enhanced a bit in 2.3, and then deprecated in 4 (how much because phones were starting to include more storage so it was felt less important, and how much because Google were trying to move away from microSD and encourage others to do so, I cannot say). So from then on manufacturers had to make a choice: did they add the feature back or not? Then with Android 6 Google added an alternative, the ability to "adopt" an SD card, i.e. format and use it as internal storage (clearly by this point they'd realised that people weren't going to abandon SD just because Google recommended it). But because this method has drawbacks too some manufacturers chose to remove it from at least some of their phones (particularly flagships).
So why do they make these choices? Frankly you'd have to ask them (assuming they'd answer, which is a bit unlikely). The one case I know the reason for is Samsung flagships: the S4 was released without this feature. Many people complained about how little space the commonest 16GB version left for the user, and Samsung replied, through support lines and even press adverts, that they should just move apps to SD - the option that didn't exist! When this became a big enough issue that the mainstream media (as opposed to tech websites) picked up on it suddenly Samsung management stopped just repeating incorrect information ever more loudly and added the feature back...
But in general leaving it out is probably a case of "it's less work for us" on the part of the manufacturer.
The other method, formatting as internal, is often removed from flagships but included in lower-end devices from the same manufacturer. I expect this is a marketing decision: if someone uses that option and doesn't use a very fast card it can slow the phone, and that customer is more likely to blame the phone than their choices. So to avoid bad publicity that might hurt sales they just remove the option.
For the specific phone you ask about I cannot say whether you can or not. These are just my thoughts on the general "why" question you asked.