In the past there was little choice of materials for aquariums.
Glass was the only thing aquariums were made of.
Even with the wider range of materials these days, some people still prefer glass.
Some pros associated with glass:

• Clearer: Some people find that glass tanks appear clearer; whereas, some acrylic appear to be cloudy.
  
• Scratch resistant: Glass tanks will not scratch as easily as the softer acrylic aquariums may.
  
• Convenient: It is easier to find glass aquariums.
  Almost all stores carry glass.
  You can even find them at yard sales or advertised in papers.
  

Some cons associated with glass:

• Heavier: Even when empty a glass tank weighs quite a bit.
  This can mean it is more difficult to move.
  You will also need to consider the weight when it comes to choosing a stand for your aquarium.
  
• Break more easily: Glass is more rigid so it can be cracked or broken easily.
  This can be an important consideration if you have young children in the house.
  
• More likely to leak: While sealants have improved over the years it is still more likely that a glass tank will have problems with the seal.
  

Some pros of acrylic:

• 17 x stronger than glass: This is especially important with larger aquariums.
  
• Impact resistant: Acrylic is softer and more flexible; it will give with a blow instead of cracking or breaking.
  
• Weighs half as much as glass: This makes it much easier to move your tank.
  Even a large tank could be moved by one person when empty.
  
• Insulates 20% better: This reduces temperature fluctuations which can make it difficult for your fish.
  It can also reduce the heating and cooling needed in your tank.
  

Some cons associated with acrylic:

• Difficult to find: While some stores now carry a line of acrylic aquariums, it may be difficult to find one in the size you need.
  You may have to consider having one custom made.
  
• Flammable: You have to be sure that no heat source is near the aquarium.
  Make sure that no candles are set close to it or floated in the aquarium.
  
• Solid support under tank required: The stand you choose for your aquarium will have to have a solid top that fully and completely supports the base of the aquarium.
  
• Special cleaning tools: You may have to get special cleaning tools that are made for use with acrylic.
  This can be expensive if you have to replace the cleaning tools you currently use.
  

Since there are good and bad points to each type of aquarium, it may end up being a matter of personal choice.
It would also make sense to take the cost into consideration.
If someone is selling a glass aquarium for a lot less than you could purchase a new acrylic tank it would be silly not to choose glass.
People with more than one aquarium may decide to try one of each type so that they can do their own comparison.