Mainly because it's a company and the fact they run funding where people think blockstream must have some private agenda or they use LN to centralize bitcoin and take profit so they can paid their investor. But the fact are :
1. Only fraction of Bitcoin developer work at Blockstream
2. Even if Blockstream were to defunct, Bitcoin and Bitcoin Core development won't be stopped as there are thousand contributor worldwide
This article isn't 100% accurate, but it sums up this topic :
https://medium.com/bitcoin-not-blockchain/the-blockstream-conspiracy-and-the-bitcoin-cash-pump-8f35fb44491aSo it's pretty much the fact that people want all developers in the Crypto space to not have the ability to benefit from their talent (in a completely separate area though, as Blockstream =/ Bitcoin Core) I think this is the correct assumption.
As people who are on the Bitcoin Core team and working for the Blockstream company, are able to separate their duties (I would hope)
