Are there any reasons not to upgrade to 200A service?
Cost
Service upgrades tend to be expensive, since it usually involves installing a new service panel.  Depending on your situation, you may find that the cost far outweighs the reward. If you already have 100 Amp service, and you haven't added on to the home, you may find the upgrade unnecessary.
Availability
The upgrade has to be supported by the existing infrastructure, otherwise the provider may charge you to upgrade their system.  Just like the wires in your home have to be sized properly to carry the load, the distribution wires also have to be capable of providing the extra power.  If the distribution system is at, or near its limit, the provider will have to make upgrades to the system before it can offer an upgrade to you.  In this case the provider will either tell you it can't offer you the service, or they'll offer the service but only if you pay extra to help offset their costs.  This is not a common scenario in most large US cities, though can still be encountered in less populated rural areas.