Because only attributive adjectives change in Gaelic to agree with the noun and predicative adjectives never do.
That means that adjective changes depending on the gender, number, and case of the noun only when it stands next to the noun to describe it, like in:
- a’ bhean bhochd – the sick woman,
- na balaich bheaga – the small boys,
but the adjective always stays in the base form when it itself complements the verb to be:
- tha a’ bhean bochd – the woman is sick,
- tha na balaich beag – the boys are small.