The answer to your question is multi-dimensional and very long. I am assuming your question applies to the US, as other countries do have unionized technology workers.
- Historically Engineers were considered white collar, and draftsman, machine operators, and assemblers were considered blue collar
- Software engineers used to be a distinct culture, different from both management (Jocks/Frat Boys), and blue collar workers - this has changed dramatically in the last 30 years and the divisions have blurred
- Early hackers were egalitarian but distrusting of central authority, they were rebels, loner rebels do not form unions.
- Ayn Rand, objectivism, etc. had a strong influence on computer scientists, engineers, etc. That influence has waned. It was once very common for engineering schools to have objectivist clubs, now they are rare.
- Tech workers, despite being pushed to ridiculous hours, rarely looked at themselves as victims in the past. Sure you didn't shower for a week, but you were making bank getting to build new and exciting things. Today the job is less exciting and you are more likely to be maintaining someone else's mess than inventing something new.
- Even if you were laid off in the 70s and 80s you felt very secure because tech was booming, you could go somewhere else. That changed in the 90s, despite the dot com boom, the balance finally shifted to where there were enough tech workers to not guarantee you would be be immediately hired elsewhere.
- Unions are widely distrusted among the US population. Yes they gave us the 40 hour work week and weekends, but they are often seen as obstruction to change not agents of change.
- A lot of people in the US simply don't like unions.
There are a lot of reasons to not like unions, but that was not your question.
I am personally in favor of unions in the private sector, but not in the public sector. I am against compulsory membership and believe unions should be more transparent.
Edit: also I was thinking about it and collective bargaining power for tech is probably even worse now than it once was because many companies could just move software engineering overseas.