While it is true that space based defenses enjoy significant tactical advantages over their planetary counterparts, most arguments in favor of them overlook a key point, which is that of cost and resources. Building orbital installations requires the movement of mass from other places in the gravity well (or the planet's surface) to construct them. The availability of both mass and energy are extremely limited in space. It is for this reason that even the largest scale projects to date in Earth-based space programs have resulted in installations only slightly larger than school buses. Even with space elevators (clearly evident in Sins) and massive advances in technology, this still remains a severe limitation. This limitation is highlighted by the small number of civilian installations or population centers on anything other than planets (Even large asteroids support only very small planets). Only the Vasari have the technology, resources or will to create space-born installations that support taxpayers, and even for them, it comes at considerable expense (population upgrades for starbases).
Planets are able to cheaply produce virtually unlimited supplies of mass and energy weapons and defenses. Relative to space construction, they are not limited by scarcity of resources, expense of construction, size of installation, size of workforce or safety considerations to the same degree. Additionally, the size of planets and their density makes them relatively safe from bombardment (which explains why it takes so damn long to take down a fully equipped world). Defenses can be placed around population and administrative centers where appropriate, but can also be installed at a significant remove from those same. They can be buried under miles of rock or camoflauged into their environment. The Cold War on Earth gives an ideal example of this situation; the ability to harden defenses was a significant part of the calculation of Mutually Assured Destruction (MAD). The fact that almost 100,000 high yield nuclear weapons were constructed to ensure deterrence is evidence of both the industrial power of planets and of the difficulty inherent in destroying one.
Planets should be more than capable of constructing powerful defenses, immense armaments stockpiles and vast energy reserves to power their resistance against orbital attackers. Even with the admittedly significant disadvantages associated with surface-to-space combat (everything from the cost of getting into orbit to firing angle limitations (you can't shoot through your own planet)), they ought to be able to put up quite a fight against most fleets that might choose to engage them. The question, then, is why don't they? I think the answer is three-fold.
First, there appears to be significant limitations on the availability and deployability of shield technology in Sins. Only the TEC have the ability to protect an entire world from bombardment, and even they are only able to do so with orbital installations. It seems clear that atmospheric interference makes it difficult or impossible for ground based installations to properly protect civilian, economic and administrative centers. The fact that shields tend to only reduce, rather than entirely block damage from weapons striking them is an added concern (see phase missiles, etc.) Frankly, as other posters have mentioned, it is difficult to protect civilian populations in the event of a direct attack. Therefore, it is imperative to prevent enemy fleets from getting close enough to inflict sustained bombardment in the first place, rather than attempt to fight them when the siege cordon is already in place. Resources are allocated accordingly.
Second and most importantly, however, is the fact that the planets we are referring to in Sins are in fact colony worlds rather than fully developed planets. Even extrapolating game time to years suggests a profound limitation on the ability of planets to become the kind of defensive powerhouses some commanders might like. While Earth might be able to sustain a massive military-industrial effort on behalf of its own defense, a two year old colony in the best of conditions is unlikely to be able to do so, even with advanced technology speeding their development and immigration sustaining population growth. The fact of the matter is that colonies expend all of their internal industrial output on colony development. This is why planets never generate metal or crystal that can be used towards fleet production. Instead, they generate wealth, and considerable amounts of it as population sizes increase. This to our interests as commanders, because if planets were wasting resources building defenses, they wouldn't be doing the important activity of generating wealth to sustain our forces. The fact that they can even fortify enough to offer some protection for populations on the surface is quite an acheivement given the difficulties facing planetary governors.
Third and finally, planets do offer a considerable contribution to gravity well defense in the form of the crews, command and energy they provide to orbital facilities and fleets. As we all know, the destruction of a planet disables all orbital installations which may have survived the bombardment. While we can assume that this loss of control is related to the loss of command and control at the colony level, it is probably also related to the loss of power and logistical support from the planet (as other posters have suggested). Wireless transmission of energy to Gauss defenses and other such installations helps keep the cost of orbital installations down (since energy can be generated cheaply on the surface) and frees up orbital resources for other activities. Only starbases have the energy reserves to supplement or replace what the planet itself can provide, and only then at considerable expense.
All that said, i think we fleet and sector commanders owe some gratitude to the base and colony commanders supporting us from the ground. They do everything they can to aid us and then some. Asking more of them is only shirking our responsibilities to protect our worlds.