In a telezoom, most people don't need constant aperture or other pro-level features, nor can they afford them (ie--the *60-250/4), so when you compare the 55-300 to other telezooms in it's class (ie--the Tamron or Sigma 70-300s), the 55-300 is a good deal. If you don't need the pro-level features, then why buy the *50-135 that doesn't have as much reach and is twice as expensive?
Each person has to decide for themselves what works best for them in terms of gear and their budget and not necessarily what everbody else is buying. If a 55-300 works better than a*50-135 for someone, then more power to them.
Heather