This is an interesting question. The role of deep brain stimulation in Parkinson's disease has increased in the last years.
Recently, de Souza et al adressed your concern and reviewed studies investigating the timing of DBS in PD. Here are their conclusion:
From the evidence available, we conclude that surgical management of
PD alone or in combination with medical therapy results in greater
improvement of motor symptoms and quality of life than medical
treatment alone. There is evidence to support the use of DBS in less
advanced PD and that it may be appropriate for earlier stages of the
disease than for which it is currently used. The improving short and
long-term safety profile of DBS makes early application a realistic
possibility.
To your question regarding why DBS is not used as first line therapy:
Well, while various studies have shown a significant reduction of dyskinesias, improvement of motor symptoms and significant improvement of quality of life when compared with best medical treatment, DBS is also associated with side effects (surgery related, device related and stimulus related).
Concerning stimulus related side effects (ie other structures than the targeted Subthalamic Nucleus or Globus Pallidus Interna get stimulated through the produced electric potential), you have to imagine, that the electrical field produced by DBS can propagate to other brain regions and may therefore lead to psychological symptoms or motor symptoms. Research on this field is ongoing but because "in vivo" testing can't be undertaken (it would not be unethical), bioengineers are creating "models" than enable to stimulate DBS electrical field distribution.
Overall, yes, DBS is promising and current debate suggests it should be used earlier. BUT our current understanding on how DBS works (on neuronal level) is still poor and further research is needed.
Sources (both are open access):
Groiss SJ, Wojtecki L, Südmeyer M, Schnitzler A. Deep Brain Stimulation in Parkinson’s Disease. Therapeutic Advances in Neurological Disorders. 2009;2(6):20-28. doi:10.1177/1756285609339382.
deSouza R-M, Moro E, Lang AE, Schapira AHV. Timing of Deep Brain Stimulation in Parkinson Disease: A Need for Reappraisal? Annals of Neurology. 2013;73(5):565-575. doi:10.1002/ana.23890.
