Assuming that the Starkiller Bases' shields were top of the line and that planetary shields are not designed to deal with hyperspace, that could be another reason why the Starkiller Base was built. We don't know how powerful planetary shields are in Episode 7, but it's
possible that in the 40+ years since the original Death Star came out someone finally developed shield tech strong enough to block a hit. It could help explain why they went that route over simply rebuilding the Death Star 2, which would have been quicker to build while still unstoppable via conventional fleets. in addition to removing the exhaust port weakness the DS2 presumably would have had a much "tighter defence" than the original in order to handle star fighters, Dodonna pointed out that the original Death Star could have been much more difficult to attack if they had viewed starfighters as a threat.
EDIT: Actually that's a good question: what do you think took more time / materials / manpower to build, the Death Star 2 or the Star Killer Base? I would place my bets on the Star Killer Base due to its sheer size, new tech, and the fact the First Order was
much smaller than the Empire, though it was carved out of a planet rather than being entirely artifical. I wonder how much more quickly the DS2 could have been built if the Emperor had decided not to make a super secret project and just built it near a major industrial world the Rebels had no chance of successfully attacking?
Finished it it today. Here's something that may be of interest to users here. I have the Kindle version, so I can't quote the exact page. It's chapter 7, the scene where Rey and Finn got the Falcon captured and think they're about to be boarded by the First Order (they get boarded by Han and Chewie instead):
She refused to accept it. "There have to be other options besides dying!"
He sighed heavily. "Sure. We could run--if the engines could be powered up. We could try and fight--if the blasters would function. We could step into the matter transporter--if such a thing existed." He shook his head dolefully.
Well, that could imply a few things:
Finn's just being sarcastic, and the transporter idea is little more than fiction in the Star Wars universe. There's a few times where I've wished I transporter myself, doesn't mean I'm likely to get one for my birthday.
The Star Wars universe has attempted at various to make a working theory as to how to operate a matter transporter, but has so far not succeeded.
The Star Wars universe has the theoretical knowledge on how a matter transporter works, but does not have the practical knowledge or tech necessary to build such a thing. Kind of like how the Federation has been unable to build a working trans warp drive even though they've been trying for nearly a century, the tech base just isn't there yet. Or like our difficulties in developing fusion power.
The Star Wars universe does in fact have matter transporters, but they are rare and/or experimental enough that most people aren't aware of them. IMO this seems the least likely scenario, but it's a big galaxy out there and I suppose that someone out there may be close / have developed a working model. After all, nobody outside of the New Order was aware of the StarKiller base or it's capabilities until it suddenly blew up the New Republic's capital planets.