Because it adds no information. It's already implied by the context. The use of the past tense contains the same information that the word "previously" does.
I must say, though, that the sentence does seem a little too terse if worded as, "We are now going into areas we couldn't go into due to lack of troops." It doesn't look right that way, because "couldn't" could be misread as being conditional rather than past tense -- the sentence will probably not be misinterpreted, but it may take another second to "parse" correctly -- so in that sense, "previously" isn't that redundant because it helps eliminate that problem. But also the use of have/had without "previously" solves that problem as well.
- Kef
I must say, though, that the sentence does seem a little too terse if worded as, "We are now going into areas we couldn't go into due to lack of troops." It doesn't look right that way, because "couldn't" could be misread as being conditional rather than past tense -- the sentence will probably not be misinterpreted, but it may take another second to "parse" correctly -- so in that sense, "previously" isn't that redundant because it helps eliminate that problem. But also the use of have/had without "previously" solves that problem as well.
- Kef