(1) FTDNA were pioneers, and IMO a little more technically astute than Ancestry
(2) Ancestry has better tools to work with searches and matches.
(3) Trust factor is less with Ancestry. Right now there is uproar in the genealogical community about their decision to discontinue support for the Family Tree Maker Software. Even with the pitchfork anger, Ancestry is ignoring us.
Ancestry certainly has more tools than does FTDNA or 23andMe. But the people in the DNA genealogy community whom I trust the most seem to think that Ancestry's tools give misleading and incorrect results in a high percentage of the cases.
For example, you might want to read some of the posts in Roberta Estes' blog, DNAeXplained, where she has very unfavorable comments about Ancestry's offerings.