Because he isn't that smart according to his actions. Defunding Obamacare was impossible and he knew that, yet he initiated a shutdown which hurt many American people, he is not fit for office. His personal agenda is ahead of the people, he solely did all this for attention, however, with that said he probably does believe in his wacky principles.
It WAS smart politically to go after obamacare. Everyone in congress, on getbig, in america KNEW it wasn't going to get defunded. Obama won that battle when the supreme court humped his leg and gave him the green light. Chunks of it are already in action. It's here to stay.
We all know that.
BUT the redmeat far-right base just LOVES a guy that is still fighting it, however futile. "I fought obamacare long after others gave up, and i'm STILL fighting it!" is VERY SMART POLITICALLY.
It's bullshit that makes you smirk if you have an IQ over 110 and you know exactly what he's doing. but guess what? The "base" of BOTH parties ain't scoring 110 on IQ tests. The extreme 10% on both sides vote using their hearts, not their brains. Every time. Like clockwork. Dems and repubs alike.
Cruz is feeding them red meat to get their support for 2016 primaries. Low-info voters in SC, FL, Iowa, etc... going after obamacare now means nothing to 80% of americans that realize this monstrosity is here to stay. But for the 10% base that decides the GOP candidate, Cruz is fighting for them.
SMART. It's very smart, politically.