The main reason goes well beyond tech companies, to all large organizations. In the workplace, leftists are more discriminatory regarding political ideology. People on the right are often willing to hire and promote people on the left, but that graciousness is not reciprocated. Leftists fire Trump supporters. Over time, this leads to complete control.

There are minor reasons too. One is that many tech workers are young people fresh from university indoctrination, but without children and mortgages. Another is that many tech workers come from outside the USA. Besides that fact that many non-USA places are more leftist, there is a selection bias caused by the fact that these people are not dedicated to staying in their own countries; they are fundamentally more globalist than the people that they leave behind.