Because half of the kids in Welsh speaking schools are being educated in their 2nd language, so they are learning a subject with very specific linguistic terms, as you say science has it's own language, via a middle language (Welsh), it is a waste of limited time and energy. Also there is a Wealth of resources (books, education videos, peer reviewed articles written in both English and other mainstream languages, where there isn't in Welsh.
In year 4 my daughter should be learning about vertices in Maths, what are the Welsh word for Vertices, vector, diagonal, magnitudes and coordinates. These are difficult concepts for an 8yo, so she has to learn and understand the words, which will be a mix of entire translations some obviously just Welsh spellings of the recognised word in the scientific community, and she has to make sense of this to read the question to do the Maths, which is what she is meant to be learning. This is too much to ask an 8yo, and it is compounded by the 'child-led' approach to Welsh Education where she can opt to stick on the green or yellow chilli level, and who can blame her, her head must be spinning!!
This stubbornness to name the problems in Welsh Education, because it offends a few Welsh speakers are destroying our childrens furtures, and it definetly is not all, I have First Language Welsh friends sending there kids to the most English medium secondary school because of their own experiences in higher education.
I am fully in favour of bilingiual education, but done in a way where it only benefits my DD, not where it fails her. But we have had that choice taken away from us, with no English medium primary schools and the only remaining predominantly English medium secondary under attack from Plaid.