why is C4H10 the simplest alkene that can have structural isomers?

Question

Because compounds wwith the same molecular formula but different structural formula are called isomers in C4H10

Answer 1

Not even close.
1. C4H10 is an ALKANE, not an alkene.
2. You gave the definition of an isomer, not an explanation of why C4H10 is the simplest alkane to have an isomer.
3. To find your answer, try drawing the structural formula for C3H8 (the next in the homologous series below C4H10); now try to draw an isomer of C3H8. You can't.

Answer 2

butane
CH3-CH2-CH2-CH3

iso butane
CH3-CH(CH3)-CH3