Its all about the $$$. Some localities feel that running their own service is better than 'farming' it out to a larger service. Others feel that a larger service has more resources available. Often what happens is that which ever way the municipality goes the same officers move from force to force. This is good as officers familiar with the area, people and conditions are available to maintain continuity of service. It is also not uncommon during these transitions for 'new' officers to do ride alongs to familiarize themselves with the area.
Leamington will either be going with Chatham-Kent or Windsor.. or Making a new "service" something along the lines of Windsor-Essex Regional Police. If Windsor takes over Leamington, it doesn't sound like they will revive their old department again, sadly.