Why is it uncommon to fill airliners from both ends?

Most of the airports on the continent are in EU member states and so they have to comply with Regulation 300/2008 on common rules in the field of civil aviation security (plus the subordinate regulations and the delegated regulations). This requires members to impose strict rules on boarding aircraft. And it goes one step further by requiring that the carriers draw up procedures and submit them for approval.

What this amounts to is that boarding a commercial flight in the EU is a controlled activity which takes place under regulatory supervision. If there is a breach of procedures, the carrier and the airport (and in big cases the member state) can be fined (that's a bad thing). If there's a grave breach, the carrier may be banned from EU destinations (that's a very bad thing).

So in order to have two separate boarding sites, the carrier would need to double their infrastructure at those sites (not to mention building them). Plus they would have to have a secondary procedure when two sites were in use and get it approved. If the carrier could satisfy the regulator with their procedures they could probably implement them. But it seems the carriers do not want to bear the extra burden of paperwork and infrastructure expense. Probably if one carrier goes through the rigmarole to have multiple boarding sites, then the others will follow by herd instinct. But the cost will be passed down the demand chain until it reaches YOU.

Some of the newer members may still be in the convergence stage (e.g., Hungary). There are 7 such states in total. When they get full membership their procedures will be compliant just like the existing member states.

Just to be complete, the procedures for boarding, getting off, transiting, deplaning, and whatever are all governed by the same regulatory framework.

This shouldn't be taken to mean that some carriers not have drawn up procedures, particularly the smaller craft that board from the apron anyway. Also, there will always be people who discount the influence of regulations as irrelevant bureaucracy, but in truth the body of airline regulations in the EU are actionable against carriers and airports alike. It is a ponderous corpus of important law and shouldn't be easily dismissed as irrelevant. Airlines are not "sovereign citizens who live outside the law", and almost every aspect of their operations is under regulator purview in one way or another. Carriers who opt to ignore them as irrelevant actually will get into trouble and face fines and other sanctions.

The alternative view is to say, There are no EU regulations that control how passengers board/deboard/etc. It's all done with by opening all the windows and doors and everyone jump out any which way they can. I'm in more of the first category, hence this answer.

The short answer ... cost. To have a jet bridge available to load from the rear doors, means building an access corridor from the main waiting area to the 2nd jet bridge, then installing a rather long jet bridge to access the aircraft.

The 2nd jet bridge needs to retract to a point to allow clearance for the wings of the aircraft as it approaches the gate. And that clearance needs to be fairly generous as big jets don't steer with absolute accuracy.

So adding up the extra space needed for aircraft clearance of the 2nd jetway, plus the width of the access corridor's building and you have taken away most of a parking slot for another jet. This means fewer gates or much more expense building a longer concourse. While this might not be an issue at a small airport, it is a huge issue at a big busy airport.

Using stairs as the rear loading method also involves more expense, airline staff to monitor the people on the ramp area, plus airport security staff. It also likely mandates a shelter of sorts that can be extended during rainy weather to protect boarding passengers. Plus there is the general inconvenience to the passengers of going down the stairs from the departure lounge to the ground level, then back up the stairs to board through the rear door. And let us not forget the gate agent's headache ... grumpy passengers who want to know why they have to take the stairs and go outside in the heat or cold or rain, while others get to walk down the jet bridge.

(I know some airports have dual jet bridges, but I don't think those are ever used for aircraft the size of a Boeing 737.)

While this is largely correct, there's an exception for every damn rule; in this case, it's the Albany International Airport in New York state, USA. The two gates there owned by Southwest Airlines (which famously uses nothing but 737s to save on operating costs) have a dual-jetbridge system that actually allows both the front and rear doors to be used for loading and unloading. You can see how the jet bridges are constructed via Google Maps aerial photography; and here's a YouTube video of the rear jetbridge retracting. I haven't seen this system, where the rear jetbridge actually goes over the wing once the plane is parked, anywhere else.

There are many airports/terminals designed to allow embarking and disembarking on narrow-body short or medium-range jets through both doors. In this case, the objective is to minimise turn-around time and reduce costs and the terminal is often a bare bones building with waiting areas at ground level so no reason to use even a single jet bridge.

And there are also many terminals with jet bridges and departing passengers on the first floor but here comfort is the main objective and having some passengers go down and up two flights of stairs would make the whole process more complex and unpleasant. It's a trade-off but building/using a second jet bridge for a small airplane also seems harder to justify.

To me, the Norwegian solution is the most surprising one: If you want to save costs and fill the airplane as quickly as possible, you always have the two-stair option but if you are ready to let at least some passengers experience the inconvenience of going up and down and walking on the tarmac, why even pay for a gate with a jet bridge?