Because conservatives need every Republican seat possible to assure a Republican takeover of the Senate.
You don't seem to grasp the reality that there will never be enough conservatives to elect 51 Ted Cruzs to the Senate. We have to put up with some establishment types to control it.
The goal is 1) win the Senate 2) Elect enough conservatives to control the agenda of the Senate and 3) elect a conservative President with a mandate.
I also suspect someone like you would be accusing Ronald Reagan of being a RINO if he were up for reelection because he granted amnesty to illegals and agreed to some tax increases.
Even principled conservatives aren't perfect. Grasp that fact or you're doing more harm than good.
Why?
Seriously, what would republican control of the senate gain us? — It's not like the republican party is powerless, were they to stand on principle they could have defunded ObamaCare, defunded the NSA, defunded the ATF, defunded the IRS, they could have kept the government shutdown
going... seriously, why should we continue to endorse/support such a feckless party?