Because the same thing has happened successfully in most other European countries. Nationalist parties talk about scary immigrants, ordinary parties tighten immigration rules, and the nationalist parties fail to gain power.

For example, Denmark created the highly criticized "Smykkelov" in 2016 which lets us confiscate any values asylum seekers have over 10.000 DKK (e.g. jewelry as the name says, but never actually used for jewelry just cash) in 2016. It has been hardly used (10 times in the first 3 years), but it had enormous press coverage. The largest left party (and the party of current PM) voted for it.

The previously largest nationalist party (DF) have never been in power, despite existing for 30 years and getting 20+% of the vote in 2015 -- at most they were a support party to the right-wing government.