I think it's more likely they simply chose to prioritise walking outside your ship over walking inside your ship.
Not abandoning the latter entirely.
Both of them seem big enough features to be the headline of their own expansion.
Walking outside just seems it has more potential for the kind of open-ended, exploration/travel/combat/missions gameplay Elite is known for and presents a lower hanging fruit in terms of technological and game design challenges. Many of those challenges were already solved when the SRV's came out.
An example of a game design challenge would be, what would you actually do while inside your ship? I'm sure they could come up with something, but it certainly would be easier to answer the same question for the vast, expansive environments outside your ship.
Easier to do + higher potential = higher priority feature. Higher priority = they release it first.
Kind of like how VR, hopefully, is just a lower priority feature and they'll eventually get around to it.
As for why it's taking them long, well why they're a small indie company. If they're too destitute to afford to support VR, what makes you think they could release ship interiors and planet surfaces at the same time? Thankfully there are developers like Boundless Dynamics who march at the forefront of technology.