The main reason is severe  hypotensive effect, which can cause shrinkage of the eye- phthysis bulbi and subsequent vision loss

True that phthysis is a risk. Other issues are; cyclodiode is a painful procedure which induces much inflammation, can lead to a sudden pressure spike (inflammatory/steroid related) prior to the anti hypertensive effect. The effect of diode laser is temporary and it has been associated with macular oedema and epiretinal membrane formation. 

Additionally, it requires para/retrobulbar anesthesia which increases the risk of wipe-out syndrom (unless you can do it under GA). This is particularly relevant for patients with advanced glaucoma. It might need to be repeated, it is a destructive procedure, coagulated ciliary body can't be restored (hence risk of hypotension if the effect is too strong). Better alternatives might be SLT or trabeculectomy as primary procedures if topical treatment is not sufficient or feasible.

I agree with previous answers, 

Thank you for your input. appreciate it

Aim of glaucoma therapy is to halt the progression of visual field loss. Control of IOP (intraocular pressure) is the means to achieve this aim. Cyclophotocoagulation is a destructive procedure. Also, in addition to its complications ,it has an unpredictable IOP lowering effect. Therefore, cyclophotocoagulation is   considered only when all other measures to control IOP fail.    

This video from EyeTube offers an interesting perspective. Explains, in part, why our collective view of diode CPC may be negatively skewed. A worthwhile watch.

http://www.eyetube.net/video/transscleral-diode-laser-cyclophotocoagulation-in-eyes-with-good-vision/

Thank you Michel. That was an interesting video looking at the other side of the coin

There are two main reasons: it is impossible to calculate the amount of effect, and second, if eye has useful vision with this procedure you are destroying the source of aqueous humor which is essential for the health of many intraocular structures

As stated above, the reluctance comes because it is irreversible and you can go too far and end up with phthisis. However, if it is staged and done incrementally, it can be safe and effective. I do it with subconjunctival anesthesia alone and patients are quite comfortable. They get quite a bit of flare postop, but surprisingly little pain. As alluded to above, I think old bad experiences with excessive cyclocryo give the cyclolaser a bad rep. I find transscleral cyclo diode laser a very good alternative in uncontrolled glaucoma where a trans pars plana tube is not possible (as a retina guy I do pneumatically stented trans pars plana Baerveldts for neovascular and complicated glaucomas). Also, Iridex is coming out with a new trans conjunctival probe to treat the pars plana with micropulsed diode laser. This, like MLT and SLT, does no damage, works, and has no risk of irreversible hypotony. 

I completely agree with the previous answers. Even with the staged trans-scleral photoablation the desireable outcome can not be predicted well.Compare to commonly used primary procedures it shows lower safety profile specially in novice hand.

In my opinion the main reason is that this procedure is dose-dependant and is not objectively controled ending in extremely treated ciliary processes- causing decreased aqueous humor production and finally loss of the eyeball as an organ, not only NLP , but also phtysis bulbi. Taken into concideration aforementioned this method is reserved for refractory neovascular glaucoma with non transparent media

Unreasonableness of the proposed energy parameters for the implementation of some methods of laser cyclocoagulation, lack of metrological support and very often low qualifications of ophthalmologists.