These are some of the reasons
- The site or area was thought to be healthy or well-situated – this is the most frequent reason. The French city of Montpellier was an early favourite with foreign travellers to the south of France from the middle ages onwards, because it was in a relatively elevated position, and compared with the surrounding land it suffered much less from malaria. There are multiple examples, especially in the 1700s and 1800s, of writers using Montpelier as a by-word for 'the most favoured spot' - for instance Inveresk ... the Montpellier of Scotland.
- implied ‘classiness’ – many of the early overseas Montpelliers, especially in the British Isles, developed into sought-after areas of fashionable towns. The name has become part of a long list of up-market names (Belmont, Fairview, etc, etc) that developers reach for when they want to give a new development a positive image
- specific associations with France - the original settler of a new Montpellier either came from this part of France, or admired the French. This explains for example the best-known US Montpelier, the state capital of Vermont.
- secondary or referred usage - (very common in US) the person naming a new town/site wanted to commemorate his/her home state: many western US Montpeliers were named by Vermont migrants, recalling the name of their state capital
- surname - occasionally, a North American Montpel(l)ier turns out to have been named for an individual of this name (obviously ultimately of French origin).
One L or two?
The one L spelling, seen in a French official decree of 1668.
For a long time, the spelling of the French city was not formally fixed. While Montpellier seems always to have been the dominant form, occasionally Montpelier is found, with just one L. Most early borrowings of the name reflect this spelling. Because Montpelier, Vermont, is one of these, it has set the standard for most other US usage.
Since the French standardised on the two-L spelling sometime in the 19th century, British and other non-US usage has tended to follow suit, but there is no hard-and-fast rule.